Bombings killed six U.S. Marines in western Iraq, the military reported Friday, edging American war deaths toward 2,000 as U.S. forces ramped up offensives countrywide in anticipation of an onslaught of insurgent attacks on Iraqis voting in next week's constitutional referendum.

In Baghdad and Basra, meanwhile, accusations of extrajudicial killings mounted against Iraq's new Western-trained, largely Shiite Muslim police force.

More than 3,500 U.S. and Iraqi troops are conducting at least two military operations -- one in the far west near the Syrian border and one in north-central Iraq -- aimed at disrupting strongholds and supply lines of Iraq's Sunni Muslim-led insurgency; another assault in the west concluded late Friday, the military said.

Military operations are expected to continue through Oct. 15, when Iraqis are to vote on a draft constitution that would allow the country to be remade into a loose federation with a weak central government heavily influenced by religious law. Iraq's newly empowered Shiite majority and Kurds back the draft, while many Sunni Arabs strongly oppose it.

The insurgency is as strong in the west as anywhere in the country, with foreign and Iraqi guerrillas using Euphrates River towns to funnel recruits, weapons and money from Syria into Iraq. On Thursday, a bomb placed on a road near the border town of Qaim killed two Marines; they had been on a logistics patrol, according to Maj. Gen. Stephen T. Johnson, the commander of western forces.

Another roadside bomb Thursday blew up a U.S. Humvee near Fallujah, 35 miles west of Baghdad, killing four Marines, Johnson told reporters in a video news conference.

The deaths brought to 1,947 the number of U.S. troops killed in Iraq since the beginning of the war in March 2003, according to a count maintained by the Pentagon.

Saleh Mutlak, a Sunni Arab leader who took part in negotiations for the new constitution, urged the United States and insurgents to stop fighting during the Muslim holy month of Ramadan, which began this week. The two sides should join in direct talks afterward, Mutlak said. It was not clear whether his statement represented the view of any insurgent faction.

In the southern city of Basra, British troops arrested an unspecified number of Iraqi police officers accused of involvement in planting roadside bombs that killed at least 14 Americans, Britons and others in recent months. The arrests reflected growing tensions between local Shiites and Western-led forces in the once-calm south.

British officials said Friday that their investigations into bombings this summer led late Thursday night to the arrests of 12 people, including Shiite militia fighters and members of Basra's mostly Shiite police force. Situated in the Shiite-dominated south, Basra has experienced comparatively few attacks by the Sunni-led insurgency. However, residents and others there have alleged that the police forces are infiltrated by former Shiite militia members and used by Shiite political leaders for extrajudicial killings and torture.

Basra's police have also been increasingly at odds with British forces in the area. Cooperation has been suspended since last month, when the British used an armored vehicle to help free two of their soldiers, who had been detained by police.

Also arrested late Thursday were fighters loyal to Moqtada Sadr, a Shiite cleric strongly opposed to the Western occupation of Iraq. In Baghdad, Sunnis mourned 22 men who family members said had been taken from their homes, tortured and killed by men wearing the uniforms of Iraq's elite Interior Ministry commandos.

The dead men -- 21 Sunnis and one Shiite -- had been taken from their homes in August, relatives said. Their bodies were discovered Wednesday, laid out in the street of a small town near the Iranian border, according to families and police.

"They were found handcuffed and tortured," said Muhammed Omran, 23, a friend of one of the dead.

Police commandos are increasingly being accused of kidnappings, killings and torture, mostly targeting young Sunni men. In August, the bodies of 37 Sunni men taken in a police sweep of a Baghdad neighborhood were found in a dry riverbed near the Iranian border.

At least seven Iraqi civilians were killed in shootings around Baghdad on Friday, and at least two bodies were found dumped here, the Associated Press reported.

Special correspondent Saad Sarhan in Najaf and correspondent Jonathan Finer in Baghdad contributed to this report.