BAGHDAD — Saboteur bombers destroyed the two minarets of a revered Shiite shrine in Samarra early today, in a repeat of the 2006 attack that shattered its famous golden dome and unleashed a wave of retaliatory sectarian violence that still bloodies Iraq. Sunni extremists of al-Qaida were quickly blamed.

The assault on the Askariya Shrine, one of the holiest in Shiite Islam, immediately stirred fears of a new round of intra-Muslim bloodshed, and prompted the 30-member bloc of radical Shiite cleric Muqtada al-Sadr to suspend its membership in Iraq's parliament, threatening a deeper political crisis.

To ward off a surge of violence, Prime Minister Nouri al-Maliki quickly imposed an indefinite curfew on vehicle traffic and large gatherings in Baghdad.

It wasn't clear how the attackers evaded the shrine's guards to mount the stunning operation, detonating the blasts around 9 a.m., and bringing down the two slender golden minarets that flanked the dome's ruins at the century-old mosque. No casualties were reported.

Policemen at the shrine were subsequently detained and will be questioned as part of the investigation, al-Maliki said.

In addition to ordering the curfew, al-Maliki's office 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.

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-Qaida to sow dissent and inflame sectarian strife among the people of Iraq."

In neighboring Shiite Iran, which has been accused of funding and arming Shiite militias in Iraq, President Mahmoud Ahmadinejad blamed U.S. forces for failing to prevent the mosque attack, and threatened to halt regional cooperation to stop Iraq's spiraling violence.

In a televised address, al-Maliki said he had ordered security forces to bolster protection of Iraq's religious shrines and mosques. The Shiite prime minister also warned against reprisal attacks.