"This isn't the first time that we have seen this kind of attack. We don't know if they are trying to inspire an uprising or a prison break," Brigadier General Mark Kimmitt told AP Radio.

All of the casualties were security detainees, Morgenthaler said, meaning they were suspected of involvement in the anti-US insurgency or of being part of Saddam Hussein's ousted regime. The prison houses some 5000 security detainees.

US Marines patrolling Baghdad discovered the area from where the mortars were fired, but the insurgents had already abandoned the area, Morgenthaler said.

The attack was the bloodiest against the sprawling prison complex in western Baghdad. In August, six security prisoners were killed in a mortar attack on the lockup, which was once Saddam's most notorious prison.

Last week, insurgents fired mortars at an Iraqi police station and a US base in the northern city of Mosul, but missed their targets and killed eight Iraqi civilians.

In Mosul today, a roadside bomb exploded as a US military convoy passed by, killing a US soldier and wounding four others, the military said. Three Iraqi civilians were also wounded, Lieutenant Colonel Joseph Piek said.

Also today, Iraqi security forces, some wearing flak jackets and carrying weapons, moved back into the besieged city of Fallujah, part of an agreement between US officials and local leaders aimed at ending hostilities. The agreement calls on insurgents to hand in their weapons and allows for the return of civilians.

US officials have warned that if guerrillas do not surrender their weapons, Marines are prepared to storm the city.

"If the peaceful track does not play itself out ... major hostilities will resume on short notice," US spokesman Dan Senor said.

Announcements on US military-run radio broadcast into the city called on residents to turn in machine guns, grenade launchers, missiles and other heavy weapons to Iraqi security forces or at the mayor's office.

Senor would not comment on whether any guerrillas had turned in weapons, but cautioned that "time is running out".

Marines were under orders not to fire on Iraqi carrying weapons but not aiming them - in case they were heading to turn them in. Until now, Marines could shoot at anyone with a weapon or wearing the black uniform typically worn by insurgents,said Captain Shannon Johnson.

One group of men were seen "actively brandishing" and loading rocket-propelled grenade launchers today, Marine Lieutenant Colonel Brennan Bryne said. Troops hit the group with mortars, killing eight and destroying three vehicles, he said.

But Fallujah was largely peaceful, with long lines of cars filled with civilians and returning Iraqi police at a US checkpoint.

Some 200 members of the Iraqi security forces had returned to their jobs by this afternoon (local time) and dozens more wearing blue uniforms and flak jackets were lined up at the checkpoint to return to the city.

Iraqi families also lined up at the checkpoint. As part of a deal announced yesterday, the US military agreed to let 50 families a day back into the city, but the lines at the checkpoint were so long today that some 150 people had to be turned away, said Captain Ed Sullivan.

Kimmitt acknowledged today that US soldiers shot and killed two Iraqis working for the US funded al-Iraqi television station a day earlier, but said the two had been filming a military checkpoint in the central city of Samarra and failed to stop despite repeated warning shots.

Cameraman Jassem Kamel, who was wounded in the attack, said that US soldiers opened fire on the car after the group had finished filming police and security posts and were driving to film the city's famous spiral minaret.

"We were not filming. We were just driving in a normal car," Kamel said.

Kimmitt said US forces fired warning shots three times toward the journalists and their vehicle.

"After more warning shots, the vehicle didn't stop and continued to approach the base's gate and were engaged with direct fire," Kimmitt said.

US officials said the incident would be fully investigated.

The deaths raise to 26 the number of Iraqi and foreign journalists and employees for news organisations killed in Iraq in the past year, according to the Committee to Protect Journalists.

Since the start of the month, at least 1,100 Iraqis have been killed in fighting, according to an Associated Press count. At least 99 US troops have been killed in action, making April the deadliest month since the US-led invasion began in March 2003.

Lieutenant General Ricardo Sanchez, the top commander of US forces in Iraq, visited soldiers outside of Najaf today and indicated that there were no immediate plans to storm the city and end a standoff with anti-American cleric Muqtada al-Sadr, who controls a large militia. Najaf is home to the holiest Shi'ite shrine.

"The issue of Sadr is bigger than Sadr. It's about the Shi'ites and the holy shrines. That's the challenge I have," Sanchez said.

Also today, US and coalition military leaders were working to fill the gap left by the decision of Spain and Honduras to withdraw their troops. Kimmitt said the existing troops could be shifted to new positions, patrol areas could be redrawn or new troops could be brought in.