(Adds highest U.S. troop levels)

By Ross Colvin

BAGHDAD, Aug 7 (Reuters) - Four more U.S. soldiers have been killed in Iraq, the military said on Tuesday, raising the U.S. death toll for the first six days of the month to 21 as thousands of troops battle militants in intense summer heat.

Off the battlefield, Iraq's crumbling national unity government was in crisis after five secularist ministers said they would boycott cabinet meetings until Prime Minister Nuri al-Maliki addressed demands they first gave him in February.

The move means that 17 ministers, nearly half the cabinet, have now quit or are boycotting government meetings. The main Sunni Arab bloc pulled out last week and ministers loyal to Shi'ite cleric Moqtada al-Sadr withdrew in April.

While Maliki went ahead with a trip to Turkey and Iran, the secular Iraqi List of former Prime Minister Iyad Allawi held a news conference in Baghdad to explain why they were boycotting the meetings, in which the embattled prime minister now only has a narrow working majority.

"In February, the List delivered a written list of proposals to the government. Since that time we have not received a reply," leading Iraqi List lawmaker Iyad Jamal-Adin said.

Washington is growing increasingly impatient with the lack of political progress by Iraq's deeply divided political parties towards national reconciliation while U.S. troops continue to die in roadside bombings, rocket and mortar attacks and shootings around the capital.

U.S. President George W. Bush has sent nearly 30,000 extra troops to help stabilise Iraq and give Maliki's Shi'ite-led government breathing space to reach a political accommodation to end the sectarian violence that has torn the country apart.

The Pentagon said on Tuesday that about 162,000 U.S. troops were now in Iraq, more than at any other stage of the war.

Eighty soldiers were killed in July, a drop from the tolls in the previous three months, which were the deadliest quarter for U.S. troops since the invasion in 2003.

August, however, is now on track to be one of the bloodiest months of the year, suggesting a resurgence in militant attacks.

CONVOY HIT

Three U.S. soldiers were killed when a roadside bomb hit their convoy south of Baghdad on Saturday, the military said, while a soldier was killed in the capital on Monday by a powerful roadside bomb of a type Washington says is being supplied to Shi'ite militias in Iraq by Iran.

Four other soldiers were killed on Monday in Diyala province, where U.S. troops have launched a summer campaign against militants using the area as a staging ground for car bomb attacks in Baghdad.

Bush has warned that August will be a bloody month for U.S. forces in Iraq as militants try to influence the debate over the war in Washington, where Democrats in Congress want troops pulled out within months.

A total of 3,682 U.S. soldiers have been killed since the U.S.-led invasion to oust Saddam Hussein in 2003.

Britain's Defence Ministry said a British soldier was killed by small arms fire in the southern city of Basra on Monday, taking to 165 the number of British soldiers killed.

While U.S. troops have recorded some successes against militants, Maliki's brittle national unity government has unravelled, dealing a blow to efforts to pass laws which Washington sees as pivotal to reconciling the warring sides.

The Iraqi List said it was not quitting Maliki's government, but the length of the boycott by its four ministers would be determined by the prime minister's response to their action.

"The ministers will continue to run their ministries and will be in contact with the presidency council," Jamal-Adin told the news conference.

He said the demands included the suspension of a committee charged with rooting out former members of Saddam's Baath party and cleansing the security forces of sectarian influences. (Additional reporting by Andrew Gray in Washington)

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