The No. 2 U.S. commander in Iraq expressed hope on Wednesday that radical Shiite cleric Muqtada al-Sadr would use his influence to stop his followers from attacking U.S. and Iraqi forces as clashes spread to the outskirts of Baghdad.

The American military said 21 suspected gunmen were killed in Shiite militia strongholds late Tuesday, while Iraqi officials said 15 civilians were among the dead, including two women.

The fighting, which began a month ago in response to an Iraqi government crackdown on militia violence, has put a severe strain on a cease-fire called in late August by al-Sadr. The anti-U.S. cleric threatened this weekend to unleash his Mahdi Army militia in an "open war" if the military operations persist.

Despite heightened rhetoric by al-Sadr and his followers, U.S. commanders have been careful not to directly link the cleric to the current fighting, instead blaming Iranian-backed Shiite fighters it claims are "special groups" criminals who have broken with his movement.

"We do not attribute what we've seen to JAM," said Lt. Gen. Lloyd J. Austin III, using the Iraqi acronym for the Mahdi Army.

But he acknowledged that al-Sadr could stop the attacks.

"We certainly hope that Sadr will choose the road of peace and responsibility," Austin, who commands day-to-day operations in Iraq, said Wednesday at a news conference.

Fierce fighting broke out late Tuesday and continued through Wednesday morning in Husseiniyah, a Mahdi Army stronghold to the north of Baghdad's embattled Sadr City district.

Seven people were killed, including two women, and 20 were wounded, including women and children, according to Iraqi police and hospital officials.

American soldiers killed six Shiite extremists Tuesday night after coming under fire as they were recovering a Bradley fighting vehicle that was stuck in the mud in Husseiniyah, Lt. Col. Steve Stover, a military spokesman, said in an e-mailed statement.

U.S. soldiers also killed 15 other suspected militants in separate attacks in Sadr City, the military said separately. The sprawling area in northeastern Baghdad has been the focus of daily clashes that broke out after al-Maliki launched the crackdown.

Iraqi officials, who all spoke on condition of anonymity because they weren't authorized to release the information, said eight civilians were killed and 44 others wounded in fighting in Sadr City.

One seriously wounded man died as an ambulance speeding him to the hospital was caught in the crossfire, and an elementary school was damaged, police said.

The clashes that have been centered in Sadr City, which has a population of some 2.5 million people, have taken a heavy toll on civilians, although the U.S. military insists it takes all possible precautions to avoid hurting innocent Iraqis.

At least 315 people have been killed in the area since the outbreak of fighting began on March 25, according to an Interior Ministry official, who spoke on condition of anonymity because he wasn't authorized to release the information.

The official said no breakdown was available for the number of militiamen, civilians and Iraqi security forces. But an Associated Press count shows at least 200 of those killed have been civilians.

In northern Iraq, meanwhile, back-to-back bombings within 30 minutes of each other killed four people and wounded 12 in the Al Qaeda stronghold of Mosul.

The first attack occurred about 11:15 a.m., when a homicide bomber pushing a cart detonated his explosives about 200 yards away from the police headquarters, killing two people and wounding six, spokesman Brig. Gen. Khalid Abdul-Sattar said.

A parked car bomb exploded about half an hour later as a police patrol passed elsewhere in the city, killing two passers-by and wounding six other people.

Wednesday's bombings were the latest in a series of attacks that have chipped away at recent security gains.

Austin, the U.S. commander, said more high-profile bombings were likely as Al Qaeda in Iraq tries to regroup after suffering a devastating blow last year when thousands of Sunni tribesmen turned against them.

Last weekend, the terror network — which is usually blamed for car bombings and homicide attacks — announced a one-month offensive against U.S. troops and U.S.-allied Sunnis.

Austin vowed "to work hard to hold our gains that we've made over the past several months."

The U.S. military also raised the death toll in Tuesday's female homicide bombing in Diyala province to 18 — 10 Iraqi civilians, a Kurd and seven Iraqi policemen. It also said two Iraqi policemen were wounded.

Iraqi police who received reports at the provincial headquarters gave a lower toll, saying that eight policemen were killed and 10 other people wounded when the woman blew herself up at the entrance to a police station in Jalula, 80 miles northeast of Baghdad.

It was the second homicide attack by a woman in as many days in Diyala, a flashpoint in the battle against Al Qaeda.