A dozen mortars rained down on an outdoor market crowded with holiday shoppers on Saturday, killing at least 18 people in a Shiite-dominated city that was the scene of a deadly market assault earlier this year, police said.

Three U.S. Marines were killed in combat Saturday in Anbar province, the military said, making October the deadliest month for American forces in Iraq this year.

The deaths raised the October toll to 78, surpassing the previous high figure of 76 in April and making October — with more than a week left — on course to be the deadliest month for American service members in two years.

The mortar attack in Mahmoudiyah came soon after bombs hidden in plastic bags left on five bicycles ripped through the market, which was crowded with shoppers ahead of the upcoming Eid al-Fitr holiday, said police Lt. Hayder Satar. Such dual attacks are frequently employed by armed groups to inflict additional damage on crowds that form after the initial bombing.

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Satar said at least 18 people were killed and 52 injured in the attack on the city about 20 miles south of Baghdad. Mahmoudiyah, a primarily Shiite Muslim city surrounded by rival Sunni communities, was the scene in July of one of the worst assaults on civilians in recent months when suspected Sunni gunmen sprayed grenades and automatic weapons fire in a market, killing at least 50 people, mostly Shiites.

Earlier, gunfights broke out in Hamza al-Gharbi, about 60 miles south of Baghdad, after a bomb exploded near the offices of the Supreme Council for the Islamic Revolution in Iraq, a leading Shiite political party that sponsors the Badr Brigades militia.

The party's supporters accused members of the Madhi Army headed by the radical anti-American cleric Muqtada al-Sadr of being behind the blast, Police Capt. Muthana Khalid Ali said. He said Iraqi army and police called for reinforcements and backup from American forces, who imposed a curfew. There was no immediate confirmation of U.S. involvement from a military spokesmen.

Father south in the city of Amarah, where the Mahdi Army briefly took control on Friday, shops and government offices reopened and Iraqi army units manned checkpoints, keeping the militia fighters off the streets.

The fighting in both cities underscored alarm about the growing influence of the Shiite militias, who are linked to political blocs wielding strong influence over the shaky four-month-old government of Prime Minister Nouri al-Maliki.

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At least two people were killed in Hamza al-Gharbi and 25 in Amarah, a city of 750,000 people at the head of Iraq's famous marshlands.

Haider Ali Abdullah said he rushed to reopen his tiny restaurant after hearing that fighting had ended in Amarah.

"We were terrified," Abdullah said by phone. "The last two days had a major effect on our lives since we depend on this business to make a living."

U.S. forces said they killed a key coordinator of foreign fighters under al-Qaida in Iraq in an early morning raid in Ramadi, on a booby-trapped building in the heartland of the Sunni insurgency west of Baghdad.

It said the man, who was not identified by name, had been a senior leader of al-Qaida in Iraq responsible for providing weapons and financing to foreign fighters in the country, as well as producing and distributing video clips and other propaganda.

Iraq's main Sunni Arab party on Saturday strongly backed the accord signed between Sunni and Shiite religious figures in Islam's holiest city, Mecca, on Friday evening.

Organizers of the Mecca meeting say they aim only to stop sectarian killings between rival Sunnis and Shiites, rather than impose a truce to halt attacks against U.S. forces in the country.

"We praise this step and call upon all Iraqis and the government to respond to this blessed event and support it," said Adnan al-Dulaimi, the head of the Iraqi Accordance Front, whose party holds 44 seats in the 275-member parliament.

He also called for a cease-fire between American forces and insurgent groups during the upcoming Eid al-Fitr holiday that marks the end of the fasting month of Ramadan.

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Al-Dulaimi's endorsement was echoed by U.S. Ambassador Zalmay Khalilzad, who issued a statement urging Iraqis to do "everything possible to stop the killing of the innocent."

The Shiite Fadhila party, a member of al-Maliki's ruling alliance, also organized a rally in support of the agreement, attended by about 2,000 people who chanted "no to terrorism, yes to the Mecca edict," and "stop the bloodshed."

Sectarian differences were exacerbated when parliament adopted a Shiite-backed law this week allowing provinces in the Shiite and oil-rich south to establish an autonomous region like the Kurdish one in the north.

Sunni Arabs and some Shiites opposed the law, arguing that federalism would lead to the eventual breakup of Iraq.

President Bush was meeting Saturday with top military officials over refining tactics after commanders said the two-month-old campaign to stabilize Baghdad had largely failed.

"The last few weeks have been rough for our troops in Iraq, and for the Iraqi people," Bush said Saturday in his weekly radio address. "The fighting is difficult, but our nation has seen difficult fights before."

In addition to the market bombing and the Marine deaths, at least 21 people were killed in violence around the country, including seven who died in a suicide bombing on a Baghdad bus and four killed in clashes Friday between Shiite and Sunni tribes just south of Baghdad. Five tortured bodies were found dumped along roads or in the Tigris River.