Baghdad ? Roadside bombers killed seven U.S. troops Saturday, four of them in a single blast near Baghdad, and an eighth soldier died of a noncombat cause – raising to 25 the number of American soldiers killed this week.

Explosions aimed at U.S. patrols that kill several troops at once are common, but the recent frequency of such large-scale attacks may signal that militants are using larger bombs or explosively formed penetrators, known as EFPs, as they fight back against a series of U.S. military operations.

The military has staged a series of counterattacks this week on roadside bomb factories and insurgent strongholds where stockpiles of explosives have been uncovered.

U.S. forces using tips from Iraqi informants raided a safe house before dawn Saturday and detained three militants with suspected ties to Iran, the military said. The operation in Sadr City, Baghdad’s main Shiite enclave, was the latest in a series of raids on targets where militiamen are believed to have ties to Iran.

The United States claims Iran is arming Shiite militias and some Sunni insurgents with EFPs, which have killed hundreds of U.S. troops in recent months.

In the deadliest attack Saturday, four U.S. soldiers were killed and an Iraqi interpreter was wounded when a bomb blew up their vehicle during combat operations northwest of Baghdad.