(Updates with Maliki condeming Basra mosque attack; curfew)

By Alister Bull

BAGHDAD, June 15 (Reuters) - All U.S. troop reinforcements heading to Iraq to help restore security have now arrived, but it could take several more months before their weight is fully felt, the U.S. military said on Friday.

The United States has sent around 28,000 extra troops to Iraq for a fresh security push, launched in mid-February, aimed at curbing sectarian killing and winning the government of Shi'ite Prime Minister Nuri al-Maliki time for political reform.

"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," U.S. military spokesman Lieutenant-Colonel Christopher Garver said.

It will take 30 to 60 days for the new arrivals, who have taken total U.S. troop levels in Iraq to 160,000, to win the confidence of residents and start getting the intelligence needed to counter insurgent and militant attacks, Garver said.

That means troops might not be operating at full capacity until August. The top U.S. military commander in Iraq, General David Petraeus, and U.S. Ambassador Ryan Crocker are due to report on the success of the security build-up in September.

U.S. President George W. Bush is under growing pressure from Congress to begin pulling troops out and end the unpopular war, which has killed more than 3,500 U.S. soldiers since the U.S-led invasion to oust Saddam Hussein in 2003.

The U.S. military said 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 U.S. troop deaths so far this month. A total of 126 were killed in May.

THREE-DAY CURFEW

Garver said the relatively low intensity of reprisals since another attack by suspected al-Qaeda militants on a revered Shi'ite shrine in Samarra could signal that the presence of more U.S. forces on the streets of Baghdad was beginning to help.

Hundreds died in sectarian retaliation in the first days after militants blew up Samarra's Golden Mosque in February 2006, and tens of thousands have perished since in the bloodshed it provoked.

On Wednesday, the mosque's minarets were destroyed.

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

The largest Sunni mosque in the Basra region in southern Iraq, the Talha mosque in Al Zubair, about 15 km (nine miles) southwest of Basra city, was blown up early on Friday, security forces said. Pictures showed that it had been reduced to rubble.

"The criminal attack targeting the shrine of Talha ... is a serious crime to inflame sectarian tension among the Iraqi people," Maliki warned in a statement.

He also declared an immediate curfew in Basra province until further notice. He had already ordered security forces to step up protection at shrines and mosques around the country.

The head of Basra's emergency security committee, Major-General Ali Hamadi, said a woman and a child were killed when shockwaves from the blast hit nearby homes.

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