The grim details of the January 20 attack were released for the first time by the US military after the new Speaker of the US House of Representatives, Nancy Pelosi, made a surprise visit to Iraq.

Pelosi pressed Baghdad to take "primary responsibility" for the country's security, even as Iraqi and US officials said they were targeting Iranian agents in Iraq under a tough new policy aimed at depriving sectarian violence of outside support.

"It makes sense that if somebody's trying to harm our troops, or stop us from achieving our goal, or killing innocent citizens in Iraq, that we will stop them," US President George W. Bush told reporters in Washington Friday.

A US military statement said the Karbala raid had been "well rehearsed", with gunmen getting past Iraqi checkpoints in "a convoy consisting of at least five sport utility vehicles."

One soldier was killed in the initial assault while four others were shot dead after being kidnapped by the insurgents, who fled with them in the SUVs.

Iraqi police later found the vehicles near the town of Al-Mahawil, east of Karbala.

"Two soldiers were found handcuffed together in the back of one of the SUVs. Both had suffered gunshot wounds and were dead. A third soldier was found shot and dead on the ground," the statement said.

"Nearby, the fourth soldier was still alive, despite a gunshot wound to the head." He was rushed to a nearby hospital but died on the way.

In an initial statement released on January 21, the military had said five US soldiers were killed and three wounded "while repelling the attack."

The Internet site icasualties.org, which tracks US losses in Iraq, identified the five as Captain Brian Freeman, Lieutenant Jacob Fritz, Specialist Johnathan Chism, Private 1st Class Jonathan Millican and Private Shawn Falter.

Saturday's statement said "an estimated nine to 12 armed militants" with US-type weapons had stormed the Provincial Joint Coordination Centre in Karbala, where they opened fire and hurled hand grenades at US troops.

After damaging three US vehicles with explosives, "the attackers broke off the assault, withdrawing from the compound with four captured US soldiers.

"The precision of the attack, the equipment used and the possible use of explosives to destroy the military vehicles in the compound suggests that the attack was well rehearsed prior to execution," US spokesman Lieutenant Colonel Scott Bleichwehl said in the statement.

"The attackers went straight to where Americans were located in the provincial government facility, by-passing the Iraqi police in the compound," Bleichwehl said.

"We are looking at all the evidence to determine who or what was responsible for the breakdown in security at the compound and the perpetration of the assault."

Security issues were high on Pelosi's agenda as she and five other US representatives met Friday in Baghdad with Iraqi Prime Minister Nuri al-Maliki and President Jalal Talabani.

"The sooner Iraqi leaders make necessary political accommodations, including amending their constitution to resolve outstanding differences among all Iraqi communities, the better the chances for ending the sectarian violence," the visiting US lawmakers said later in a joint statement from Washington.

In their several meetings in Baghdad, the US envoys stressed their belief that Iraqis must rapidly assume responsibility for security in the country.

"The delegation's view is that American forces should quickly begin the transition from a combat role to one focused on training, counter-terrorism, force protection, and controlling Iraq's borders," their statement said.