BEIRUT — Mortar shells crashed into an outdoor cafe at Damascus University on Thursday, killing at least 10 students in the deadliest of a rising number of mortar attacks in the heart of the Syrian capital.

The strikes have escalated as rebels fighting to topple President Bashar Assad try to enter the city, terrifying civilians whose support the opposition needs to advance its cause.

It was unclear who fired the rounds. The government blamed "terrorists," its blanket term for those fighting Assad's regime. Anti-regime activists accused the regime of staging the attack to turn civilians against the rebels.

Mortar strikes on Damascus are relatively new in Syria's crisis, which began in March 2011 with protests calling for Assad's ouster, then evolved into a civil war. The U.N. says more than 70,000 have been killed in the conflict.

Since last month, mortar shells have hit previously safe parts of the capital with increasing frequency. The near-daily strikes have frightened residents, and many have begun to avoid open areas and put plastic on their windows to help block flying glass from an explosion or shrapnel.

Some shells appear aimed at government targets, such as one of Assad's palaces and the general command of the Syrian army. Others have hit near civilian targets, including the Sheraton Hotel and a soccer stadium, both on the city's west side. Mortar shells also have struck in areas to the east, like the Christian neighborhood of Bab Touma.

Thursday's strike was the deadliest yet.

State-run Al-Ikhbariya TV showed video of the university cafe where blood pooled on tiles and plastic chairs and pens and eyeglasses littered the area. Later video showed people being treated in a hospital, including a woman with white bandages around her head and a man whose back was peppered with shrapnel wounds.

The dining facility belongs to the Faculty of Architecture in Damascus's central Baramkeh district.

State TV said 15 people were killed in the strike, but the official news agency, SANA, put the death toll at 10 and said dozens were wounded. It also reported three other mortar strikes nearby.

The opposition activist group, the Britain-based Syrian Observatory for Human Rights, put the death toll at 13.

Similar mortar attacks on Tuesday killed at least three people and wounded dozens. Six people were killed by mortar shells in different parts of the city on March 11.

"No one anywhere in the world can imagine a more criminal act than this," SANA quoted Amer al-Mardini, the president of the university, as saying. He said he hoped the wounded would heal quickly and "resume their studies as soon as possible."

Anti-regime activists accused the regime of launching the attack to tarnish the opposition's image.

Elizabeth O'Bagy, who studies the Syrian rebels at the Institute for the Study of War, said it was not possible to determine who was behind the attack, but it appeared to fit the regime's pattern of escalation. In other aspects of the war, such as the use of airstrikes or Scud missiles, the regime has gone from trying to target rebels to more indiscriminate attacks on civilians, she said.

"Because of the fact that it does follow regime behavior, it is more likely to be a regime attack," she said, while acknowledging it could also have been a rebel misfire.

Rebels have established footholds in a number of Damascus suburbs but have only been able to push into limited areas in the south and northeast parts of the capital. The government has retained its grip of downtown Damascus, although the mortar strikes have deepened fear among many residents that they will soon see the violence that has damaged many other Syrian cities.

Thursday was not the first time Syria's universities have been targeted. On Jan. 15, twin blasts hit Aleppo University, killing more than 80 people. The opposition said the regime had bombed the university, while the government accused rebels of striking it with rockets.

Also Thursday, Ghassan Hitto, the newly elected prime minister of the main opposition bloc, said he was reviewing candidates for a planned rebel interim government. It will be a service-oriented administration with nine to 12 ministries and will be based inside Syria, Hitto said during a meeting with Syrian expats in Qatar.

In Washington, State Department spokesman Victoria Nuland said the head of the opposition Syrian National Coalition, Mouaz al-Khatib, has decided to continue serving until his six-month term ends in May. Al-Khatib resigned on March 24, citing frustration with the group and its level of international support. The Coalition rejected his resignation.

Syria's conflict threatens to destabilize neighboring countries, where more than 1 million refugees have fled to escape the violence.

In Jordan, on Syria's southern border, a riot broke out Thursday in a refugee camp after Jordanian authorities refused to let buses full of refugees return to Syria because of violence over the border. U.N. refugee liaison Ali Bibi said it was unclear how many refugees were involved in the melee at the Zaatari camp, but no one was injured.

To the north, Turkey denied reports that it was deporting hundreds of Syrian refugees for rioting on Wednesday in a camp in Akcakale after a fire killed a 7-year-old child. A camp official said local authorities identified 300 people involved in the uprising and prepared to deport them, but the government stopped them.

A Foreign Ministry official said 100 refugees asked to leave the camp and return to Syria on their own.

The U.N. refugee agency did not confirm the reports, but said it was concerned about possible deportations of refugees.

In Israel, on Syria's southeastern border, the military said it was beefing up medical teams along the border because of several cases of wounded Syrians crossing the frontier for medical care. Eleven Syrians have been treated in Israeli hospitals, including one who died from his wounds on Wednesday, a military official said. Others returned home after their conditions improved.

All officials spoke on condition of anonymity because they were not authorized to brief reporters.

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Associated Press writers Albert Aji in Damascus, Syria; Suzan Fraser in Ankara, Turkey; Bradley Klapper in Washington; Josef Federman in Jerusalem and Dale Gavlak in Amman, Jordan, contributed to this report.

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  • March 2011: 120

    Thousands of Syrians rally to show their support for President Bashar al-Assad, who is facing unprecedented domestic pressure amid a wave of dissent, in Damascus on March 29 2011. (ANWAR AMRO/AFP/Getty Images)

  • April 2011: 820

    A protestor burns a portrait of Syrian President Bashar al-Assad during a demonstration after Friday prayers on April 29 2011 in Istanbul against the regime of al-Assad and the deadly crackdown on opposition protests. (BULENT KILIC/AFP/Getty Images)

  • May 2011: 850

    A veiled woman takes part in a protest calling on Syria's President Bashar Assad to step down, in front of the United Nations headquarters in Amman, on May 21 2011. (KHALIL MAZRAAWI/AFP/Getty Images)

  • June 2011: 1,000

    Syrian refugees arrive to a makeshift camp in the northern city of Idlib, in Syria, on June 13 2011. (MUSTAFA OZER/AFP/Getty Images) <em><strong>CORRECTION:</strong> An earlier version of this caption placed the city of Idlib in Turkey. Idlib is in Syria. </em>

  • July 2011: 1,600

    Thousands of pro-regime Syrians wave their national flag and portraits of President Bashar al-Assad during a rally in Damascus on July 17 2011. (LOUAI BESHARA/AFP/Getty Images)

  • August 2011: More than 2,000

    People hold pictures of Syrian president Bashar al-Assad and fallen Libyan strongman Muammar Gaddafi as they take part in a demonstration gathering activists opposed to Syria's regime of President Bashar al-Assad on August 28 2011 at Taksim Square in Istanbul. (BULENT KILIC/AFP/Getty Images)

  • September 2011: 2,700

    A national flag hangs on a statue of Syria's late president Hafez al-Assad at the entrance of the flashpoint city of Homs on August 30 2011, as rights activists reported widespread anti-regime protests across Syria on the first day of the feast marking the end of Ramadan. (JOSEPH EID/AFP/Getty Images)

  • October 2011: 3,000

    Supporters of Syrian President Bashar al-Assad wave Syrian flags during a pro-regime rally in Damascus on October 12, 2011. Assad's regime is facing international pressure amid a violent crackdown on anti-government protests that broke out in March across Syria. (LOUAI BESHARA/AFP/Getty Images)

  • November 2011: More than 4,000

    Syrian soldiers carry on November 26, 2011 the coffin of a comrade reportedly killed in an ambush by an armed group in the flashpoint Syrian city of Homs. (AFP/Getty Images)

  • December 2011: More than 5,000

    Free Syrian Army captain identified as Ahmed al-Arabi sits in a safe house near Wadi Khaled on the Lebanese-Syrian border on December 30 2011. (Si Mitchell/AFP/Getty Images)

  • January 2012: 7,100

    Free Syrian Army fighters take position in a house on the Lebanese-Syrian border prior to a nighttime operation on January 2 2012. (Si Mitchell/AFP/Getty Images)

  • February 2012: 7,500

    A Turkish journalist in Ankara, holds pictures of two journalists, French photojournalist Remi Ochlik and Sunday Times correspondent Marie Colvin, killed in an alleged rocket attack by Syrian regime forces against a makeshift opposition media center in the besieged city of Homs in Syria on February 22 2012. (ADEM ALTAN/AFP/Getty Images)

  • March 2012: More than 8,000

    A defected Syrian soldier, now a member of the Free Syrian Army, stands outside a mountain outpost near the village of Janudieh in the northern province of Idlib on March 20 2012. (STR/AFP/Getty Images)

  • April 2012: Close to 9,000

    Syrians pray over the bodies of Syrian violence victims at a funeral in the northwestern town of Kafr Zeta on April 10, 2012. (AFP/Getty Images)

  • May 2012: More than 9,000

    A general view shows the Syrian flag flying next to destruction in the Bab Amro neighbourhood of Homs on May 2 2012. (JOSEPH EID/AFP/GettyImages)

  • June 2012: 14,000

    A Syrian man carries a wounded girl next to Red Crescent ambulances following an explosion that targeted a military bus near Qudssaya, a neighbourhood of the Syrian capital, on June 8, 2012. (AFP/GettyImages)

  • July 2012: 19,000

    Members of Jihadist group Hamza Abdualmuttalib train near Aleppo on July 19, 2012. Rebels seized control of all of Syria's border crossings with Iraq on July 19. (BULENT KILIC/AFP/GettyImages)

  • August 2012: 23,000

    A Syrian boy whose family has been displaced due to fighting between rebel fighterws and Syrian government forces is seen near the Syrian border with Turkey on August 25, 2012. (ARIS MESSINIS/AFP/GettyImages)

  • September 2012: 30,000

    A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in the northern city of Aleppo on September 14, 2012. (MARCO LONGARI/AFP/Getty Images)

  • October 2012: 36,000

    A Syrian boy plays on a destroyed tank near the rubble of a mosque that was destroyed during fighting between Syrian rebels and regime forces in the northern city of Azaz on September 23, 2012. (MIGUEL MEDINA/AFP/GettyImages)

  • November 2012: 40,000

    A walnut tree stripped of its branches stands in the rubble of the Kalat al-Numan citadel, originally built during the Roman era some 2000-years-ago, after allegedly being bombed several times by the Syrian air force on November 18, 2012, in Maaret Al-Numan in southern Idlib province. (John Cantlie/AFP/Getty Images)

  • December 2012: 60,000

    Smoke rises in the Hanano and Bustan al-Basha districts in the northern city of Aleppo on December 1, 2012 as fighting continues through the night. (Javier Manzano/AFP/Getty Images)

  • January 2013: 65,000

    A boy plays with a balloon in a Syrian refugees camp in Azaz, near the Turkish border, on January 10, 2013 after snow falls. (EDOUARD ELIAS/AFP/Getty Images)

  • February 2013: More than 70,000

    Syrian Zakia Abdullah sits on the rubble of her house in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. (Pablo Tosco/AFP/Getty Images)