"Everyone is here on the ground now. But obviously, the troops that have just got here are going to take some time to integrate into their battle space and get to know their counterparts," Garver said.

It will take 30 to 60 days for the new arrivals, who have taken total US troop levels in Iraq to 160,000, to win the confidence of residents and start getting the intelligence needed to counter violence, he added.

That means troops might not be operating at full capacity until August.

General David Petraeus, the top US military commander in Iraq, and Ryan Crocker, the US ambassador, are due to report on the success of the security build-up in September.

US deaths

The US military also said that four soldiers were killed on Thursday, three when their vehicle was hit by an explosion in northern Kirkuk province. The fourth was shot dead in Diyala province north of Baghdad.

There have been 42 US troop deaths so far this month, down sharply from the 126 killed in May.

Washington is braced for more casualties in the coming months.

John Negroponte, the US deputy secretary of state, told reporters at the briefing with Crocker: "We expect that the fight for security will get harder over the coming months as we engage an increasingly desperate enemy".

Negroponte was ending a three-day visit to Iraq to press for more progress on reforms aimed at reconciliation, including a long-awaited law to share oil revenues.

 Curfew

Garver said the relatively low intensity of reprisals since an attack on a Shia shrine in Samarra on Wednesday could be a sign the presence of more US forces on the streets of Baghdad was already helping.

Prompt calls for restraint from Iraq's political leaders and Shia clerics, including from the firebrand leader Moqtada al-Sadr, have helped contain retaliatory strikes so far to scattered attacks against Sunni mosques.

"I have been struck over the last day by the very serious and measured way - not only the Iraqi government but senior political and clerical figures -- have dealt with this," Crocker, said on Thursday.

Garver attributed the low level of retaliatory attacks to a three-day curfew in Baghdad that ends on Saturday and the fact that the Iraqi police and army were doing a better job.

 Basra mosque

The largest Sunni mosque in the Basra region in southern Iraq was, however, blown up and destroyed early on Friday morning, a senior security officer said.

"All members in charge of security at the mosque have been arrested," Major General Ali Hamadi, head of the provincial Basra emergency security committee, said.

 But there were no casualties in the attack on the Talha mosque in Al Zubair, a town around 15 km southwest of Basra city, in keeping with the pattern since Wednesday of reprisals against property, not people.