Five American soldiers died in Iraq, the U.S. military announced Friday, a day after extremists fired shells into Baghdad's Green Zone during a visit by the State Department's No. 2 official.

The prime minister imposed an indefinite curfew on Basra, Iraq's second largest city and gateway to the Persian Gulf, after bombers leveled a Sunni shrine just outside the city.

Gunmen armed with rocket-propelled grenades attacked the Talha Bin al-Zubair shrine about 13 miles outside Basra late Thursday, damaging the building, police said. They returned early Friday, planting bombs inside the structure that destroyed it, police said. No injuries were reported.

Gen. Ali al-Mussawi, a top Basra security official, said the bombers were disguised as cameramen who asked guards for permission to film inside the shrine. Minutes after they left, a huge explosion rocked the building, destroying the dome and minaret, he said.

The guards were detained afterward for questioning, al-Mussawi said.

Talha Bin al-Zubair was one of the Prophet Muhammad's companions and commands high respect among Sunnis. The shrine was renovated in late 1990s, during Saddam Hussein's rule. Sunni pilgrims from India, Pakistan and Turkey frequently visit the shrine.

Prime Minister Nouri al-Maliki's office issued a statement calling the bombing of the Sunni shrine another of the "crimes aimed at sowing sedition and inflaming sectarian strife among the people."

He said those who are attacking places of worship were "the enemies of God, the homeland and the people and they should not be tolerated."

Three of the U.S. soldiers were killed when a bomb exploded near their vehicle Thursday during operations in Kirkuk province, in northern Iraq, the U.S. military said in a statement. Another soldier was wounded in the blast.

A fourth soldier was killed by small arms fire the same day in Diyala province, northeast of Baghdad, another statement said. And another soldier died Wednesday in a non-combat related incident, which the military said it was investigating.

A Baghdad-wide clampdown continued Friday, with a curfew still in place two days after suspected Al Qaeda bombers blew the minarets off a sacred Shiite shrine and stoked fears of a bloody sectarian backlash.

At least four Sunni mosques were attacked within hours of the Shiite shrine blasts in Samarra on Wednesday, and police in Basra reported four people killed in retaliatory violence there.

Thursday's barrage of rockets and mortars included one that hit on a street close to the Iraqi parliament less than a half hour before U.S. Deputy Secretary of State John Negroponte passed nearby.

The attack again showed militants' resilience — including their ability to strike the heavily protected zone — despite a U.S.-led security crackdown across the city that began four months ago. But officials paid much closer attention to any signs that Shiites could unleash another wave of retaliation against Sunnis for the explosions at the Askariya mosque compound in Samarra.

The first attack on the site in February 2006 sent the country into a tailspin of sectarian violence that destroyed Washington's hopes of a steady withdrawal from Iraq. On Wednesday, bombers toppled the two minarets that stood over the ruins of the mosques famous Golden Dome about 60 miles north of Baghdad.

The U.S. ambassador to Iraq, Ryan Crocker, echoed Washington's claim that the latest attack was the work of Al Qaeda.

"I just don't think there's any doubt that it was Al Qaeda that first struck the Askariya in February 2006, and the method this time was very similar to that — (explosive) charges very carefully placed to devastating effect," Crocker told a group of reporters.

Negroponte called the Samarra attack a "deliberate attempt by Al Qaeda to sow dissent and inflame sectarian strife among the people of Iraq."

The U.S. military issued a statement Thursday saying Iraqi forces had arrested the commander and 12 policemen responsible for security at the shrine, which holds the tombs of two revered 9th century Shiite imams. It was not immediately clear whether the police arrested are suspects in the attack or held for questioning.

Meanwhile, insurgents linked to Al Qaeda released a videotape showing the execution-style deaths of 14 Iraqi soldiers and policemen after the expiration of a 72-hour deadline for the Iraqi government to meet their demands.

In a statement that preceded the video footage, the Islamic State of Iraq said its religious court "ruled that God's verdict should be implemented against the renegades" after its demands were not met. In an earlier video, the group demanded the release of all female prisoners in Iraqi prisons.

The killings took place in what looked like a rural area, with a grass field and several tall eucalyptus trees. A small wooden shack stood in the background.

The authenticity of the 1 1/2-minute video could not be verified, but it appeared on a Web site commonly used by Islamic militants and carried the logo of the Islamic State of Iraq's media production wing, al-Furqan.

The U.S. soldier deaths announced Friday brought to at least 3,520 the number of American military personnel who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count. The figure includes seven military civilians. At least 2,889 died as a result of hostile action, according to the military's numbers.