SAMARRA, Iraq (Agencies) - A series of blasts have destroyed two minarets at an already damaged Shia shrine in the northern Iraqi town of Samarra.

The explosions were heard in the vicinity of the Al-Askari shrine mosque at about 9:00am (0500 GMT) on Wednesday.

"The explosion targeted the two golden minarets. They have been damaged ... This is a criminal act which aims at creating sectarian strife," Saleh al-Haidari, the head of the Shia endowment in Iraq, said.

The two minarets toppled even as security forces were guarding the holy site.

Nuri al-Maliki, Iraq's prime minister, has blamed the attack on Al-Qaeda and supporters of Saddam Hussein, Iraq’s former president.

The Al-Askari mosque was already severely damaged after a February 2006 attack blamed on Al-Qaeda fighters.

Prime minister quickly imposed an indefinite curfew on vehicle traffic and large gatherings in Baghdad. Before the curfew took hold, arsonists set fire to a Sunni mosque in western Baghdad, police said. Iraq's top Shia cleric, Grand Ayatollah Ali al-Sistani, called on "believers to exercise self-restraint and avoid any vengeful act that would target innocent people or the holy places of others." Iran says enemies of Iraqi unity responsible for Samarra bombing

Iranian Foreign Ministry spokesman Mohammad-Ali Hosseini vehemently condemned the bombing of the Al-Askari shrine, calling it a criminal, anti-Islamic act.

The Islamic Republic of Iran condemns this hideous, anti-Islamic act and views such actions as part of the vicious plot of the enemies of Iraqi national unity meant to spark sectarian and ethnic disputes in Iraq, Hosseini said.

He also criticized the occupiers for their negligence in regard to the security situation.

The Foreign Ministry spokesman advised the Iraqi nation to adopt conscious patience and to make a vigilant response to these inhumane actions in order to prevent the architects of these heinous deeds from realizing their insidious goals.

The Iranian Parliament speaker, the Qom Seminary, and many other Iranian and Iraqi officials and organizations also condemned the terrorist acts.

It wasn't clear how the attackers evaded the shrine's guards to mount the stunning operation and bring down the two minarets that flanked the dome's ruins at the century-old mosque, AP reported.

No casualties were reported.

Policemen at the shrine were subsequently detained and will be questioned as part of the investigation, al-Maliki said. Later, the Interior Ministry said members of "a terrorist group" had been arrested and were being interrogated. The statement did not elaborate.

An official close to the prime minister, citing intelligence reports and speaking on condition of anonymity because of the sensitivity of the information, said the bombing was likely the work of Al-Qaeda, whose militants have recently moved into Samarra from surrounding areas.

In a conference call with reporters, Brig. Gen. Kevin J. Bergner was asked about unconfirmed reports of a skirmish among Iraqi security forces before the attack, which may have been somehow related to the bombing.

"It's unclear at what point relative to the explosions that happened, but that's exactly what the (Iraqi) investigation will build the best possible summary of," the U.S. military spokesman said.

In a nationally televised address, al-Maliki said he had ordered security forces to bolster protection of Iraq's other religious shrines and mosques.

His office also said he met with the U.S. commander, Gen. David Petraeus, and U.S. Ambassador Ryan Crocker to ask that U.S. reinforcements be sent to Samarra, 60 miles north of Baghdad, and that U.S. troops in the capital go on heightened alert.

A few hundred U.S. soldiers are stationed around Samarra to provide security, although they rarely enter the shrine's perimeter and leave protection of the mosque to Iraqi forces. The U.S. command had no immediate comment on military moves. Crocker and Petraeus later released a statement calling the attack and "act of desperation" and "a deliberate attempt by Al-Qaeda to sow dissent and inflame sectarian strife among the people of Iraq." Lt. Col. Christopher Garver, another U.S. military spokesman, said the command was "obviously very concerned about this and our primary goal is to prevent any violence of the kind that broke out after the last bombing."