Two U.S. Marines were killed and 22 wounded — two of them critically — in fighting in western Iraq, the U.S. military said Saturday. It was the biggest number of American casualties reported from a single engagement in weeks.

A U.S. statement said the casualties were suffered Thursday as a result of "enemy action" in Anbar province but gave no specific location or details of the fighting.

One Marine was killed "at the scene of the attack," the statement said. Another Marine died at a medical facility in Taqqadum, it added.

Eight of the wounded were flown to the main U.S. hospital in Balad. Two were listed in critical condition and six were reported as stable, the statement said. The others were taken to a U.S. clinic at Camp Fallujah, where four were hospitalized for observation.

"Our hearts go out to the families of the dead and wounded Marines," said Marine spokesman Lt. Col. Bryan Salas. "Our wounded Marines are receiving the best care available, and we look forward to their speedy recovery."

U.S. casualties have begun to rise this month following a sharp drop in March, which saw the lowest number of American dead in Iraq since February 2004. Last month, 31 U.S. service members died in Iraq, but fatalities in April have already passed 40.

Meanwhile, dozens of Iraqi police remained missing and nine were dead after insurgents ambushed their convoy Thursday evening as they left a U.S. base where they had picked up new vehicles, Iraqi and U.S. officials said.

Brig. Gen. Abbas Maadal complained that the Americans refused to allow the police to spend the night at the base, just north of the capital. But U.S. spokesman Lt. Col. Barry Johnson said no such request had been made and that the Iraqis had not asked for American troops to guard the convoy.

The attack, the deadliest against police here in months, began about 7:30 p.m. Thursday as a convoy of 109 police was traveling through a sparsely populated area near the Taji base heading back to Najaf, 100 miles to the south, Maadal said.

Police heard cries of "Allahu akbar," or God is great, and "long live jihad" broadcast by loudspeaker from a nearby mosque, Maadal said. Suddenly insurgents, including some women, opened fire and triggered a roadside bomb.

Maadal said 37 policemen returned to Najaf late Friday and about 20 more were en route. About 40 remained unaccounted for. At least nine policemen were killed and three of the 12 vehicles were heavily damaged, Johnson said. One insurgent was wounded and five were arrested, he added.

Although no U.S. troops were with the Iraqi convoy when it came under attack, Johnson said American forces responded with helicopter gunships and ground troops.

"Once the attack occurred we did respond," he said. "We helped engage and brought this situation under control."

A U.S. patrol had passed along the same route shortly before the Iraqis and called for help when they heard the firing, Johnson said.

It was unclear why the insurgents allowed the Americans to pass by without attacking them. In recent months, insurgents have shifted their attacks to Iraqi forces, which have less firepower than the Americans.

The overwhelming majority of police in Najaf are Shiites, and the area where the attack occurred is populated mostly by Sunnis.

Sectarian violence has worsened in Iraq since the Feb. 22 bombing of a Shiite shrine in Samarra. At least 11 people were killed Friday, including four who died in a pair of roadside bombings outside two Sunni mosques in Baqouba, 35 miles northeast of Baghdad, police said.

One civilian died when a suicide bomber targeted a British patrol south of Basra, wounding four Britons, police and British authorities said. Two brothers were also gunned down in front of their elderly mother, who was wounded when assailants stormed into their home in a mostly Shiite area of Baghdad, police said.

In the northern city of Mosul, at least seven people were wounded in another suicide car bomb attack on a police station, police said. Police saw the vehicle coming and fired at the driver, preventing him from entering the compound, an official said.

The others died in bombings and a shooting in Baghdad and Mosul, police said.

Delays in forming a new national unity government four months after parliament elections has sharpened sectarian divisions.

Sunni and Kurdish opposition to the Shiite choice of Prime Minister Ibrahim al-Jaafari for another term has blocked progress toward a new government.

Leaders of the Shiite alliance, the dominant bloc in the legislature, said they would attend Monday's parliament session, called to break the political logjam. Shiite politicians had earlier suggested they would boycott the session unless the dispute over al-Jaafari as well as other political posts that require parliamentary approval were resolved first.

But Shiite leaders said Friday they would attend even if no agreement had been reached on al-Jaafari or the other posts.

"We will meet Saturday and Sunday to discuss the matters of the prime minister nomination and the distribution of key posts," said Sabah al-Saedi, a Shiite politician. "We are going to attend Monday, regardless of what happens at the internal meetings."

Ridha Jawad Taqi, a leading figure of the biggest Shiite party, also said the alliance plans to attend Monday's meeting.

In an interview Friday with a British television station, al-Jaafari repeated that he would not step down.

"I was the legitimate and democratic choice," he told Britain's Channel 4 News. "I wouldn't have accepted the responsibility if I thought it was against the will of the people. I don't see how I could repay my people's faith in me by letting them down."

Although the parliament session may produce no deals at all, it is seen as a sign that the parties are committed to forming a unity government. Boycotting it would make the Shiites appear obstructionists.

Voters chose the 275-member assembly on Dec. 15, but the legislature met briefly only once last month. The lack of progress has frustrated Iraqis, especially as steady violence — much of it sectarian — continues to claim hundreds of lives and threatens to push the country into a large-scale civil war.