The U.S. military says American troops have killed three civilians, including a woman, during a raid on a village in central Iraq.

This is the second time in two days the U.S. military has admitted killing civilians during raids against suspected terrorists.

Local Iraqi forces say the three were members of the same family, and were killed in their home in Al-Dawr east of Tikrit. At least one child was wounded.

The U.S. military says troops came under small arms fire when they entered the house and they returned fire in self-defense. Officials say the incident is under investigation.

Also Tuesday, Iraqi officials say local security forces found a mass grave containing about 50 bodies near the town of Samarra, north of Baghdad. Police say some of the bodies were badly decomposed, while others appeared to have been killed more recently.

In a separate development, Iraqi police in Taji say a suicide bomber killed two local Sunni militia fighters allied with U.S. forces. Witnesses say the attacker detonated his explosives near a convoy led by a local tribal leader.

On Monday, U.S. officials said a U.S. air strike had killed nine civilians and wounded three on Saturday near Iskandariyah, 50 kilometers south of Baghdad.

The killing of Iraqi civilians by U.S. soldiers and American security contractors has strained relations between Baghdad and Washington.

Critics say American troops and security contractors use excessive force and cause unnecessary casualties. The U.S. military says insurgents deliberately use civilians as human shields.

Some information for this report was provided by AFP, AP and Reuters.