Three of the 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.

All were assigned to Task Force Lightning, and their names were withheld until their families could be notified.

A curfew remained in place in the capital two days after suspected al-Qaida bombers blew the minarets off a sacred Shiite shrine in Samarra and stoked fears of a bloody sectarian backlash.

Violence appeared mostly in check Friday, but police said unknown bombers leveled a Sunni shrine near Basra, Iraq's second-largest city.

Gunmen armed with rocket-propelled grenades attacked the shrine in suburban Zubair late Thursday, partially damaging the building, police said. They returned early Friday, planting bombs inside the structure and exploding it completely, police said. No injuries were reported.

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 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-Qaida.

"I just don't think there's any doubt that it was al-Qaida 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-Qaida 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-Qaida 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 one and a half 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.