An arson attack on a bank killed five people on Monday in a southeastern Iranian city where a mosque bombing days earlier killed 25, state media said.

The attacks were both in the restive city of Zahedan, which sits at a crossroads between Afghanistan, Pakistan and Iran and has seen a sudden explosion of violence in the run-up to June 12 presidential election.

The mosque bombing on Thursday was claimed by a Sunni militant group Jundallah, or God's Soldiers, which Iran says has links to Al Qaeda. The group has been fighting a low-level campaign against Iran's Shiite leadership for years.

State-owned Press TV said the arson attack targeted the Mehr Financial and Credit Institute, linked to the paramilitary Basij militia which is often involved in crackdowns on dissidents.

The state news agency said the city was now calm and police had arrested suspects.

Three men convicted of involvement in the mosque bombing were hanged in Zahedan on Saturday. Clashes erupted Sunday in the city after rumors that a local Sunni cleric had been attacked. On Friday, gunmen fired on President Mahmoud Ahmadinejad's campaign office in Zahedan, injuring three people.

Pakistan's ambassador to Iran was summoned to the Iranian Foreign Ministry over the bombing, the state news agency reported. Two Pakistani officials said Monday that Iran had partially closed a border crossing between the two countries.

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.

The country's Supreme Leader, Ayatollah Ali Khamenei, accused enemies of promoting sectarian conflicts in Iran. Though he did not name any country, the use of the term "enemy" by Iranian officials is usually a reference to the U.S.

"Enemies were trying to create chaos but all people should remain aware," Khamenei said Monday on state radio, adding that the enemy was targeting national unity in the country.

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 6 million of Iran's 70 million people.

Jundallah, has carried out bombings, kidnappings and other attacks against Iranian soldiers since the early 2000s to press its campaign for more rights for impoverished Sunnis under Iran's Shiite government.

Iran says the group operates across the border in Pakistan, a source of concerns for the two governments which cooperate closely on the problem.

The region's Sunni discontent has led to sectarian rioting and clashes in the past.

The Sunnis are from Iran's ethnic Baluchi minority, a community also found over the borders in neighboring Pakistan and Afghanistan.

Iran has repeatedly accused the U.S. of backing militants including Jundallah specifically and ethnic opposition groups to destabilize the Iranian government.

The militant group was behind 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. The seven were released a month later, apparently after negotiations through tribal mediators.

Adding to the region's lawlessness, the crossroads between the three countries is also a key smuggling point for narcotics. It is scene of frequent clashes between police and drug gangs.

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 militants have been blamed for bombings in the southwestern city of Ahvaz — including blasts in 2006 that killed nine people. Some 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 a southeastern airport minutes after takeoff when a homemade bomb was found aboard, said state television, in an incident a security official called a sabotage.