BAGHDAD — Three separate attacks in Baghdad killed four U.S. soldiers and at least 11 civilians, U.S. and Iraqi officials said today.

Three of the soldiers died after their Humvee was hit with an explosively formed penetrator, a type of bomb that the U.S. alleges Iran has been supplying to Shiite militias. Iran denies the accusation.

AP Television News video of the bombing Tuesday in the predominantly Shiite Mashtal neighborhood of eastern Baghdad showed the twisted wreckage of the Humvee burning wildly as soldiers hosed it down with water. Two soldiers were wounded in the attack.

Another soldier was killed and two wounded in combat Tuesday west of the capital, the U.S. command said.

Elsewhere, a roadside bomb rocked an eastern Baghdad Shiite neighborhood early this morning, killing at least 11 people and injuring 19 others when it exploded next to buses used by morning commuters, police officials said.

Blood stained the ground around a small crater caused by the bomb, which went off in the predominantly Shiite neighborhood of Baladiyat.

Nine people were killed instantly by the blast, according to police, while a medic in nearby Kindi hospital said two others died there shortly afterward from their injuries.

It was not immediately clear who was responsible, but the blast came on the fringes of Sadr City, al-Sadr's stronghold.

In a pre-dawn raid in Karbala today, U.S. forces captured an Iraqi believed to be working as the local contact to the Iranian Revolutionary Guards Corps's elite Quds Force to supply Shiite militias with Iranian-made weapons, said U.S. Army Maj. Winfield Danielson III.

The suspect is also believed to have helped transport Iraqis to Iran for "terrorist training," Danielson said in an e-mail. The military said it is believed that he is "closely linked to individuals at the highest levels" of the Quds Force.

U.S. forces were led to the suspect, whose name was not released, by information from prisoners, Danielson said.

Ground troops confiscated computer equipment, communication devices, miscellaneous documents and photographs, the military said.