BAGHDAD, Iraq - A car bomb exploded at a police checkpoint in western Iraq today, and a police spokesman said that "many" Iraqi security force members were killed or injured. Al-Jazeera television reported six dead and nine hurt.

The attack in Baghdadi, located 120 miles northwest of the capital, was the latest strike in an insurgent campaign against Iraq's fledgeling security forces following the U.S.-led assault on Fallujah earlier this month. Dozens of policemen, national guards and government soldiers have died in attacks throughout Iraq over the past two weeks.

Police Lt. Mohammed al-Fehdawi said U.S. troops immediately sealed off the road following the car bombing.

"Many policeman and National Guardsmen were killed, but we don't have a figure yet," he said.

Elsewhere, gunmen stormed a police station west of the city of Samarra late Sunday, a spokesman said. The attackers, who faced no opposition, looted the armory and commandeered several police cars before leaving the area.

U.S. troops arrived at the station today and arrested two dozen officers, the policeman said.

In Basra, British and Iraqi troops were deployed around the headquarters of the Iraqi National Guard southern regional headquarters after the chief staff, Brig. Gen. Diaa al-Kadhimi, refused to accept an order from Baghdad to remove him from his post, Iraqi officials said.

Al-Kadhimi was to have been replaced by the national guard chief in Amarah, Salah al-Maliki. The standoff was continuing this afternoon.

South of Baghdad, U.S. and Iraqi forces launched raids Sunday that killed 17 suspected insurgents, Iraqi police said. The raids included a dawn speedboat assault by U.S. Marines and British and Iraqi troops on suspected insurgent hideouts along the Euphrates River, British media reported.

The speedboat assault was the biggest operation of its kind so far in Iraq, with 130 troops racing up the Euphrates River in boats armed with machine guns and grenade launchers, British media reported.

Troops were targeting an area south of Baghdad, where Sunni insurgents have taken control of a string of towns and cities between the capital and the Shiite Muslim shrines of Najaf and Karbala.

However, the raid produced only a few small weapons caches and documents believed buried by a Saddam loyalist, British media said.

U.S. Marines, Iraqi commandos and British troops launched an offensive known as "Operation Plymouth Rock" in the area last Tuesday as a follow-up to the assault on Fallujah.

On Sunday, a statement posted on an Islamist Web site in the name of al-Qaida in Iraq, led by Jordanian terror mastermind Abu Musab al-Zarqawi, claimed responsibility for killing 17 members of Iraq's security forces and a Kurdish militiaman in Mosul, where insurgents rose up this month in support of guerrillas facing a U.S.-led assault in Fallujah.

The claim could not be independently verified but the style of writing appeared similar to other statements by al-Zarqawi's group, which is responsible for numerous car bombings and beheadings of foreign hostages in Iraq.

At least 50 people have been killed in Mosul in the past 10 days. Most of the victims are believed to have been supporters of Iraq's interim government or members of its fledgling security forces.

Separately, al-Zarqawi's group claimed it detonated a car bomb near a U.S. military convoy in the Hamam al-Alil area, near Mosul. It said the blast destroyed an armored vehicle and damaged another.

Although the claims were not verifiable, they raised fears that al-Zarqawi's organization had spread to Mosul, Iraq's third-largest city, 225 miles north of Baghdad. At least 43 suspected insurgents have been arrested as part of an ongoing operation to re-establish control of Mosul, a military statement said.

Al-Zarqawi's group, formerly known as Tawhid and Jihad, was believed to have been headquartered in Fallujah, the Sunni Arab insurgent bastion 40 miles west of Baghdad, before U.S. and Iraqi forces overran the city this month.

Al-Zarqawi and the city's two major Iraqi insurgent leaders, Sheik Abdullah al-Janabi and Omar Hadid, apparently escaped the onslaught and remain at large. Before the assault, U.S. intelligence officers speculated that al-Zarqawi would try to relocate to Mosul if he lost his base in Fallujah.

Meanwhile, three truck loads of humanitarian aid from the Iraqi Red Crescent Society arrived Sunday afternoon in Fallujah. Supplies of food, water, blankets and medicine are finally flowing into Fallujah three weeks after the U.S.-led offensive which captured the city from Sunni Muslim insurgents.

Red Crescent official Saeed Ismaeel Haqi said he would be meeting with U.S. officials to see what could be done to speed the delivery of supplies to civilians either still in the city or villages around the U.S. security cordon where people from the city fled when the assault began Nov. 8.