BEIRUT — The leader of Lebanon's Hezbollah militant group said Tuesday that Syrian rebels will not be able to defeat President Bashar Assad's regime militarily, warning that Syria's "real friends," including his Iranian-backed militant group, were ready to intervene on the government's side.

In Damascus, a powerful bomb ripped through a bustling commercial district, killing at least 14 people and bringing Syria's civil war to the heart of the capital for the second consecutive day.

Hezbollah, a powerful Shiite Muslim group, is known to back Syrian regime fighters in Shiite villages near the Lebanon border against the mostly Sunni rebels fighting to topple Assad. The comments by Sheik Hassan Nasrallah were the strongest indication yet that his group was ready to get far more involved to rescue Assad's embattled regime.

"You will not be able to take Damascus by force and you will not be able to topple the regime militarily. This is a long battle," Nasrallah said, addressing the Syrian opposition.

"Syria has real friends in the region and in the world who will not allow Syria to fall into the hands of America or Israel."

Hezbollah and Iran are close allies of Assad. Rebels have accused them of sending fighters to assist Syrian troops trying to crush the 2-year-old anti-Assad uprising, which the U.N. says has killed more than 70,000 people.

Deeper and more overt Hezbollah involvement in the Syrian conflict is almost certain to threaten stability in Lebanon, which is sharply split along sectarian lines, and between supporters and opponents of Assad. It also risks drawing in Israel and Iran into a wider Middle East war.

Nasrallah said Tuesday there are no Iranian forces in Syria now, except for some experts who he said have been in Syria for decades. But he added: "What do you imagine would happen in the future if things deteriorate in a way that requires the intervention of the forces of resistance in this battle?"

Hezbollah has an arsenal that makes the group the most powerful military force in Lebanon, stronger than the national army. Its growing involvement in the Syrian civil war is already raising tensions inside the divided country and has drawn threats from enraged Syrian rebels and militants.

Nasrallah also said his fighters had a duty to protect the holy Shiite shrine of Sayida Zeinab, named for the granddaughter of Islam's Prophet Muhammad and located south of Damascus.

He said rebels have captured several villages around the shrine and have threatened to destroy it.

"If the shrine is destroyed things will get out of control," Nasrallah said citing the 2006 bombing of the Shiite al-Askari shrine in the Iraqi city of Samarra. That attack was blamed on al-Qaida in Iraq and set off years of retaliatory bloodshed between Sunni and Shiite extremists that left thousands of Iraqis dead and pushed the country to the brink of civil war.

While there has been growing speculation about Hezbollah's role in the conflict next door, the violence inside Syria has raged on, including in the capital, where a bomb on Tuesday struck the Marjeh neighborhood, a busy commercial area near the Old City of Damascus.

The state news agency said 14 people were killed and 103 wounded in the attack.

A day earlier, Syrian Prime Minister Wael al-Halqi narrowly escaped an assassination attempt after a car bomb targeted his convoy as it drove through a posh Damascus neighborhood. The bombings appear to be part of an accelerated campaign by opposition forces to hit Assad's regime in the heavily defended capital.

"I heard a very loud bang and then the ceiling collapsed on top of me," said Zaher Nafeq, who owns a mobile phone shop in the Damascus Towers, a 23-floor office building near Tuesday's explosion. He was wounded in his hand and his mobile phone shop was badly damaged in the blast.

There was no immediate claim of responsibility, but car bombs and suicide attacks targeting Damascus and other cities that remain under government control have been claimed in the past by the al-Qaida affiliated Jabhat al-Nusra – one of scores of rebel factions fighting to oust Assad.

The target of Tuesday's attack was not immediately clear, although the explosion took place outside the former Interior Ministry building.

Inspecting the site of the blast, Interior Minister Mohammed al-Shaar, who himself escaped a car bomb that targeted his convoy in December, told reporters the back-to-back attacks in the capital were in response to the "victories and achievements scored by the Syrian Arab Army on the ground against terrorism."

In recent weeks, government troops have overrun two rebel-held Damascus suburbs and a town outside the capital. They also have captured several villages near the border with Lebanon as part of their efforts to secure the strategic corridor running from Damascus to the Mediterranean coast, which is the heartland of the president's Alawite sect, an offshoot of Shiite Islam.

As the regime has pushed back against opposition fighters, it has come under allegations of using chemical weapons. Damascus denies the charges, saying Syrian rebels are trying to frame it.

In Washington, President Barack Obama signaled Tuesday he would consider U.S. military action against Syria if "hard, effective evidence" is found to bolster intelligence that chemical weapons have been used in the civil war.

He added, however, that while there is evidence that chemical weapons were used, "we don't know when they were used, how they were used. ... We don't have a chain of custody that establishes" exactly what happened.

The White House said last week that intelligence indicates the Syrian military has used the deadly nerve agent sarin on at least two occasions.

Speaking to reporters, Obama reiterated that he needed more certainty before acting, but said that if it is determined that the Assad regime used chemical weapons "we would have to rethink the range of options that are available to us."

In Tehran, Iranian Foreign Minister Ali Akbar Salehi said that use of chemical weapons in Syria would also be a "red line" for Iran, but suggested rebel forces should be investigated rather than the Islamic Republic's allies in Damascus.

In the latest alleged attack, activists in the town of Saraqeb in northern Idlib province claimed the government bombed the town late Monday with chemical agents. It said the attack caused respiratory problems and other symptoms among a few residents that it claimed were consistent with a chemical attack.

The Britain-based Syrian Observatory for Human Rights said it was unable to confirm the purported use of chemical agents.

The Syrian state news agency offered a different narrative, saying "terrorists" brought bags of an unknown white powder to Saraqeb and opened them, causing respiratory problems among those exposed. It said the terrorists then transported the injured to Turkish hospitals to "accuse the Syrian armed troops of using chemical weapons."

Border authorities in Turkey decontaminated a group of Syrians wounded in the Saraqeb attack and hospital staff treating them wore protective equipment, according to an aide to the governor of Turkey's Hatay province, which borders Syria.

The aide, who spoke on condition of anonymity citing government rules that bar civil servants from speaking to journalists, told AP there was no indication that chemical weapons were used in the Saraqeb attack.

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Associated Press journalists Barbara Surk, Bassem Mroue and Ryan Lucas in Beirut, and Suzan Fraser in Ankara, Turkey contributed to this report.

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