BAGHDAD, June 21 (Reuters) - Two U.S. soldiers were killed and four wounded in an attack on their vehicle southwest of Baghdad as thousands of troops swept around Iraq's capital in a coordinated operation against militants, the military said on Thursday..

The combined U.S. and Iraqi army operations signal a new phase in Washington's Iraq strategy. The military is going on the offensive to crush al Qaeda and other militants, who have defied a major crackdown to secure Baghdad by hiding out in the lush green farmlands and towns bordering the Iraqi capital.

U.S. authorities also said five Iraqi civilians died and six wounded when an insurgent's rocket destroyed a building in the Al Mansour district of Baghdad, from about 10 miles (16 km) away.

"This attack was not aimed at any specific targets. It is most likely a way to incite sectarian violence," said Captain Aquila Knopf, an intelligence officer.

At least 46 militants have been killed so far, 13 wanted individuals captured and a large quantity of weapons seized in the current offensive.

South of the capital, 17 boats were destroyed to disrupt militant operations on the Tigris River. North of Baghdad, 10,000 mostly U.S. troops supported by attack helicopters, Stryker and Bradley armoured fighting vehicles are entering the third day of an offensive against al Qaeda militants in volatile Diyala province.

The U.S. military launched the operations after the fifth and final reinforcing brigade of U.S. troops arrived in Iraq, completing its troop "surge" or build-up.

U.S. President George W. Bush has sent 28,000 extra soldiers to help curb sectarian bloodshed and buy Shi'ite Prime Minister Nuri al-Maliki time to reach a political accommodation with disaffected minority Sunni Arabs, who are locked in a cycle of violence with majority Shi'ite Muslims.

The four-month-old security crackdown is focused on Baghdad, epicentre of the violence. While U.S. and Iraqi forces have reduced the levels of bloodshed in the capital, militants have shifted the fight to other provinces such as Diyala.

Al Qaeda fighters have used Diyala as a launchpad for large-scale suicide and car bomb attacks on the capital, such as the one that struck a Shi'ite mosque in central Baghdad on Tuesday, killing 87 and wounding over 200.

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