DUBAI, United Arab Emirates — An arson attack on a bank killed five people Monday in an Iranian city bordering Pakistan where authorities have struggled to quell unrest since a deadly mosque bombing last week claimed by Sunni militants.
The violence in Zahedan, which sits near the borders of Afghanistan and Pakistan, highlights the complex tensions in one of Iran's most restive and lawless regions _ crisscrossed by drug trafficking routes and hit by sporadic attacks from Sunni guerrillas fighting for greater autonomy from Iran's Shiite leadership.
It also adds pressure on authorities to bring unrest under control in the area before the June 12 presidential election.
State-owned Press TV said arsonists targeted the Mehr Financial and Credit Institute, linked to the paramilitary Basij corps that is often involved in crackdowns on dissidents. The state-run Islamic Republic News Agency said the city was calm by sundown and police had arrested suspects.
It was the latest flare-up of violence since the bombing of a Shiite mosque on Thursday, which killed 25 people and brought a swift backlash from authorities: Three men convicted of links to the attack were hanged Saturday.
The bombing was claimed by a Sunni militant faction called Jundallah, or Soldiers of God, believed to be linked to wider networks across the border in Pakistan. The group has been fighting Iran's Shiite leadership for years and has carried out attacks that include bombings and kidnappings. Iran's government accuses it of links to al-Qaida.
Iran has also repeatedly accused the United States of backing militants including Jundallah and other ethnic opposition groups to destabilize the government. The claims have added to the acrimony between Iran and the U.S. over issues like Tehran's nuclear program and support for militant groups despite the Obama administration's outreach.
Clashes erupted Sunday in Zahedan after rumors that a local Sunni cleric had been attacked. On Friday, gunmen fired on President Mahmoud Ahmadinejad's campaign office in the city, injuring three people.
The tensions also threaten to strain relations with between Iran and its eastern neighbors Pakistan and Afghanistan shortly after a three-way summit in Tehran late last month in which leaders pledged more cooperation.
Iran's Foreign Ministry summoned Pakistan's ambassador to protest the mosque bombing. Two Pakistani officials said Monday that Iran had partially closed a border crossing with Pakistan.
Qamar Masood, a senior official in Baluchistan province on the Pakistan side of the border, said the crossing at Taftan had been closed for trading but that foot traffic was still being allowed.
In Tehran, Foreign Minister Maouchehr Mottaki claimed Jundallah's reputed leader, an Iranian Sunni named Abdulmalik Rigi, was building closer ties to "foreign forces" in Afghanistan, which would suggest possible ties to Taliban backers such as al-Qaida.
Mottaki said Pakistan's President Asif Ali Zardari has cooperated with Iranian authorities to fight the militants.
Iran has deep interests in both countries _ with cultural and linguistic links to western Afghanistan and plans with Pakistan for a key gas pipeline to lucrative subcontinent markets.
Alireza Nourizadeh, a researcher at the London-based Center for Arab-Iranian Studies and an expert in Sunni-Shiite affairs, said he believes the mosque attack was meant as retaliation for pressure on Sunni groups in Zahedan since attacks blamed on Jundallah escalated in late 2006.
He said Iranian authorities also have increased surveillance and controls in the Zahedan area to fight drug trafficking from Afghanistan and Pakistan, which has disrupted the traditional flow of commerce and families across the Baluchistan region that spans the border areas of the three countries.
"This attack on the mosque and the other violence has little to do with the elections, but it is part of a longer confrontation in the area in which Sunnis feel increasingly pressured by Shiiite authorities," he said.
Nourizadeh worried that the sectarian tensions could widen to other areas of southeastern Iran with Sunni communities.
"I'm really worried that this could flare into something bigger unless both Sunni and Shiite leaders can put a lid on it quickly. It's a dangerous moment for this part of Iran."
Iran's southeast _ a region of salt flats and treeless arid hills _ is just one of the Middle Eastern flashpoints between Sunnis and Shiites that range from the tiny Gulf nation of Bahrain to the sectarian meltdown in Iraq following the U.S.-led invasion in 2003.
Zahedan is the capital of the large Sistan-Baluchistan province, home to a million of Iran's Sunni Muslim minority. Sunnis are believed to make up some 5 million of Iran's 70 million people.
Jundallah has carried out bombings, kidnappings and other attacks against Iranian soldiers and other forces in recent years, including a car bombing in February 2007 that killed 11 members of Iran's elite Revolutionary Guards near Zahedan.
Jundallah also claimed responsibility for the December 2006 kidnapping of seven Iranian soldiers in the Zahedan area. It threatened to kill them unless members of the group in Iranian prisons were released. The seven were released a month later, apparently after negotiations through tribal mediators.
The area is also a key smuggling point for drugs _ mainly opium poppies _ and is the scene of frequent clashes between police and traffickers.
Iran has faced several ethnic and religious insurgencies that have carried out sporadic, sometimes deadly attacks in recent years _ though none have amounted to a serious threat to the government.
Besides the violence in the southeast, ethnic Arab Shiite militants have been blamed for bombings in the southwestern city of Ahvaz _ including blasts in 2006 that killed nine people. Iranian Kurds based in northern Iraq have also stepped up incursions into Iran.
Late on Saturday, an Iranian airliner was also forced to return to the Ahvaz airport minutes after takeoff when a homemade bomb was found aboard, said state television, in an incident a security official called a sabotage.