American forces broke up a Shiite militant cell believed to be smuggling an armor-piercing Iranian weapon responsible for killing an increasing number of Americans and Iraqis, the military said. Separately, the U.S. announced the deaths of five American soldiers and an Iraqi interpreter.

Roadside bombs were responsible for three of the American deaths announced Friday and have long been the No. 1 killer of U.S. and Iraqi forces in Iraq, but the use of the Iranian explosively formed penetrators, or EFPs, is rising. The weapons, which hurl a fist-sized lump of molten copper, can pierce even U.S. armored vehicles newly designed to deflect roadside bombs.

In Sadr City, the Baghdad slum that is home to Shiite militias allied with radical anti-American cleric Muqtada al-Sadr, coalition raids rounded up 16 suspected members of a militant cell that brought in the Iranian weapons, as well as militants seeking terrorist training, the U.S. said. Intelligence reports also indicate the cell is linked to kidnappings in Iraq, the statement said.

The military said over the last six months U.S. forces have found and destroyed four caches of Iranian-linked weapons around Mahmoudiya, a mostly Shiite enclave surrounded by Sunni-dominated areas about 20 miles south of Baghdad. Sectarian violence and attacks by militias and insurgents on American and Iraqi forces are common in the area.

Iranian officials deny importing weapons or militants into neighboring Iraq, but The Washington Post reported Friday that attacks in Iraq involving Iranian-made EFPs reached a record high last month.

Quoting Lt. Gen. Raymond T. Odierno, who oversees day-to-day U.S. military operations in Iraq, the paper said the number of attacks with the projectiles rose to 65 in April, most of them in predominantly Shiite eastern Baghdad. Officials said the projectiles are used almost exclusively by Shiite fighters against U.S. military targets.

EFPs were first reported used in Iraq in 2005 against British forces in the south, but have grown increasingly common, primarily in Baghdad.

The Post quoted Odierno as saying that before April, the month with the greatest number of projectile attacks was December 2006, with 62. It said the use of projectile weapons has risen over time as other types of bombs have become less effective against added U.S. armor.

Overall attacks using roadside bombs doubled in Iraq from 2006 to 2007 and number about 1,200 a month, the Post said. They cause roughly 70 percent of the casualties suffered by U.S. troops, the paper said.

Roadside bombings on Friday killed an American soldier and, separately, five Iraqi policemen. Elsewhere in the capital on Thursday, roadside bombs killed two U.S. soldiers and an Iraqi interpreter. Two other American soldiers were killed Thursday in fighting in Anbar province.

Seprately, seven bodies were found floating in the Diyala River in Baqouba, 35 miles northeast of Baghdad, and snipers were preventing police and medical teams from recovering from the remains along with other bodies spotted in recent weeks from the waterway, police said.