BAGHDAD, April 28 (UPI) -- Four U.S. soldiers were killed Monday in rocket attacks in Baghdad in fighting between U.S.-backed Iraqi forces and Shiite militiamen, military officials said.

Three soldiers were killed in eastern Baghdad, and a fourth was killed in the western part of the city, The Washington Post (NYSE:WPO) reported, citing a U.S. military spokesman.

The fatalities made Monday one of the deadliest days for U.S. troops in Iraq since March, when Iraqi Prime Minister Nouri al-Maliki began an offensive against Shiite militias, the newspaper said.

Forty-four U.S. troops have died in Iraq in April, said iCasualties.org, an organization that tracks military deaths. That is the highest monthly total since September, the Post said.

Earlier, U.S. military officials said at least 36 people died in several violent incidents during the weekend around Baghdad. U.S. and Iraqi forces killed 22 Shiite militia fighters during fighting in Baghdad.

The fighting began when a "large group of criminals" assaulted Iraqi and U.S. soldiers Sunday at a security checkpoint, CNN reported Monday.

A female suicide bomber attacked another checkpoint, killing at least three Iraqis and wounding 14 others, officials said.

In Baghdad's Sadr City area, at least eight people died and 44 others were wounded in weekend fighting, a U.S. military official said. Seven Shiite militia members were among the dead.

An Interior Ministry official said police discovered a grave containing the remains of 50 people north of Baquba. The graves were in an area that once was an al-Qaida stronghold, officials said.