BAGHDAD — An unmanned U.S. drone fired two Hellfire missiles at militants attacking Iraqi soldiers in a Shiite militia stronghold in the southern city of Basra today, killing four of the gunmen, the military said.

In Baghdad, clashes between U.S.-backed Iraqi troops and Shiite militiamen in the Sadr City district killed two men and injured 18 other people, police said today.

The airstrike in Basra occurred about 1 a.m. after militiamen attacked an Iraqi army patrol with rocket-propelled grenades on the eastern side of the Hayaniyah district, the U.S. military said. A vehicle suspected of containing more weapons and ammunition also was destroyed.

The area has seen some of the fiercest fighting since a government offensive against the militias in Basra began March 25.

In Sadr City, a police officer said those injured in gun battles Tuesday included three women and three children. Sadr City is a stronghold of the Mahdi Army militia of anti-American cleric Muqtada al-Sadr. It is also home to an estimated 2.5 million Shiites.

The officer, who spoke on condition of anonymity because he was not authorized to speak to the media, said sporadic shooting was still going on and it was too dangerous to venture out on the streets.

The ferocity of the Shiite militia response to the government crackdown has surprised Iraqi security forces — which are dominated by Shiites — raising doubts about whether the Iraqis could handle an all-out war without U.S. help.

The New York Times reported that an 80-strong company of Iraqi soldiers abandoned their positions Tuesday night in Sadr City, leaving a crucial stretch of road undefended for hours despite pleas by American soldiers in the area for them to stay.

The Iraqi company leader, who was identified as Maj. Sattar, and his troops complained that they were short of ammunition and overall poorly equipped to battle the militias and had no means to communicate directly with the U.S. troops positioned behind them, according to the newspaper.