Four U.S. soldiers were killed in Iraq over the weekend, the military said on Monday, raising the number of American troops killed this month to over 100 and making April one of the deadliest of the war for U.S. forces.
The toll could increase the pressure on U.S. President George W. Bush, who is fighting a plan by Democrats to set a timetable for withdrawing U.S. forces from Iraq.
Bush has vowed to veto a war spending bill from Democrats that requires combat troops to begin withdrawing by Oct 1. The Democratic-controlled Congress plans to send the bill to Bush on Tuesday.
The U.S. military said three soldiers had been killed by a roadside bomb in eastern Baghdad on Sunday. An Iraqi interpreter was also killed. Another soldier was killed by small arms fire in eastern Baghdad on Saturday.
U.S. and Iraqi forces launched a major security crackdown in Baghdad in mid-February that is seen as a final attempt to halt Iraq's plunge into all-out civil war between majority Shi'ites and once-dominant minority Sunni Arabs.
U.S. commanders acknowledge that the offensive, which has led to the deployment of thousands of extra troops on the streets, has increased the risk of military casualties.
Before the announcement of the latest deaths, the independent icasualties.org Web site had put the number of U.S. troops killed in Iraq in April at 99. Around half have been killed in and around Baghdad.
More than 3,330 U.S. troops and many tens of thousands of Iraqis have been killed since the U.S.-led invasion in 2003.
In fresh violence, eight gunmen were killed in a U.S.-Iraqi operation in Baghdad on Sunday, the U.S. military said, in what some witnesses described as a clash with the Mehdi Army militia of anti-American Shi'ite cleric Moqtada al-Sadr.
The U.S. military said in a statement one Iraqi soldier was killed in the incident in the Shi'ite Kadhimiya district. It denied reports from witnesses that U.S. forces had entered a mosque and an office run by Sadr.
The statement gave no affiliation for the gunmen, but some witnesses said they were Mehdi Army militiamen. It also did not say if U.S. troops were involved in any exchange of fire.
If the gunmen were Mehdi Army, the clash would mark the heaviest between U.S. forces and the Mehdi militia in Baghdad since the start of the Baghdad crackdown.
Under Sadr's orders, the Mehdi Army has been keeping a low profile during the offensive.
Sources in Sadr's political movement gave conflicting reports, with some saying the clash involved members of the organization, while others said they were civilians.
A formal investigation also began on Monday to decide if a U.S. Army officer accused of "aiding the enemy" while he ran a U.S. detention centre in Iraq should face a court-martial.
Lieutenant-Colonel William Steele, commander of the 451st Military Police Detachment, ran the detention facilities at Camp Cropper, near Baghdad international airport, where insurgents and former senior aides to Saddam Hussein are held.
He is charged with fraternizing with a detainee's daughter, having an improper relationship with a translator, providing unmonitored mobile phones to prisoners, unauthorised possession of classified information and keeping pornographic videos.