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 gunbattles 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.
It added that an elite Iraqi unit was rushed in and began to fight its way north with the help of the Americans.
The U.S. military said the report was factual and the Baghdad command would address the issue.
Lt. Col. Steve Stover, a military spokesman in Baghdad, called it "a snapshot of one area where U.S. soldiers are in close support of their Iraqi counterparts" and stressed that it is a new army and Iraqi soldiers and national police are taking casualties daily in fighting in other areas.
"This is one company-sized unit, part of a recently formed Iraqi army battalion," Stover said, adding that 65 other Iraqi battalions were operating in Baghdad with "varying degrees of experience and capability."
"The older units are able to conduct independent counterinsurgency operations and others obviously need more work, better leadership and more experience," he said in an e-mailed statement.
In other violence Wednesday, a mortar shell slammed into a house in eastern Baghdad, killing at least three civilians and wounding three others, police said.
Gunmen also opened fire on a minibus near Muqdadiyah, 60 miles north of Baghdad, killing 2 women and wounding 3 men, police said.
Also Wednesday, the U.S. military said two Marines were killed by a roadside bomb in western Anbar province. The statement said the blast occurred on Sunday while their vehicle was under attack by enemy fighters.
The two deaths raise to at least 4,036 the number of U.S. military members who have died since the war started in March 2003, according to an Associated Press count.
The bloodshed came a day after nearly 60 people died in a series of bombings in four cities in northern and central Iraq that were blamed on al-Qaida in Iraq.
The bombings struck directly at U.S. claims that the Sunni insurgency is waning and being replaced by Shiite militia violence as a major threat.
Meanwhile, the Iraqi government said 33,121 detainees have been released since a general amnesty went into effect in February.
Spokesman Abdul-Sattar al-Berqdar said judicial reviews of thousands of other cases were continuing. He said that so far the reviews had identified 9,726 detainees who would not be released because they did not fall within the amnesty.
Associated Press writer Kim Gamel contributed to this report.