Two homicide car bombers attacked a market and a police checkpoint on the outskirts of Ramadi on Monday, killing 13 people and dealing a blow to recent U.S. success in reclaiming the Sunni city from insurgents.

A mortar attack also killed five people and wounded two others in Baiyaa, a religiously mixed neighborhood in western Baghdad that has seen a recent rise in sectarian tension. Four Iraqi troops were killed in separate attacks in Baqouba, northeast of the capital.

The violence came a day after roadside bombs killed eight American soldiers, including six who died in a single blast in the surrounding province of Diyala. and two in separate attacks in the capital. The mounting U.S. casualty toll highlights the dangers facing troops as they take to the streets more as part of a security crackdown in the Baghdad area.

The Islamic State of Iraq, an Al Qaeda front group, on Monday claimed responsibility for the attacks that killed the soldiers and a Russian photographer in Diyala. It also said it was behind a suicide car bombing against Iraqi police in the northern city of Samarra.

The first attack Monday targeted a public market about noon northwest of Ramadi, 70 miles west of Baghdad, killing eight civilians, said Col. Tariq Youssef, the security supervisor of Anbar province.

About 15 minutes later, police at a nearby checkpoint spotted a second car bomb and shot at the driver, but prevent him from detonating the vehicle, Youssef said. That attack killed five people, including two police officers, Youssef said.

The U.S. military has struggled for years to secure Ramadi, the capital of the insurgent stronghold of Anbar province.

The city has shown recent signs of calming, especially as Sunni tribes have allied against Al Qaeda, and military units have moved off the major bases and established smaller U.S.-Iraqi posts in the most violent areas downtown. But insurgents are fighting back for control.

"They committed this crime because we have identified their hideouts and we are chasing them," Sheik Jabbar Naif al-Dulaimi, a police officer and a member of the alliance known as the Anbar Salvation Council.

Violence also has surged north of Baghdad, where militants have fled the security crackdown in Baghdad that began on Feb. 14.

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The attacks in Baqouba, a Sunni insurgent stronghold northeast of Baghdad, began before dawn when a booby-trapped house exploded during a raid, killing two Iraqi soldiers and wounding three. About six hours later, gunmen ambushed a police station elsewhere in the city, killing two officers and wounding two others, police said.

The bullet-riddled body of a policeman bearing signs of torture also was found outside the northern city of Kirkuk.

Isurgents accuse Iraqi security forces and police of collaborating with U.S.-led forces and the Iraqi government, highlighting the challenges in preparing them to take over their own security so that U.S. and other foreign troops can go home.

Underscoring the dangers, a funeral procession was held in the northern city of Samarra for the city's police chief who was killed the day before along with 11 other officers in a bold daylight ambush suicide car bombing and shooting attack.

AP Television News footage showed tearful Iraqi police in blue uniforms carrying banners and marching as the coffin was covered with an Iraqi flag and borne through the city in a white pickup truck.

The car bomber detonated his payload as he came under fire just inside a checkpoint outside the police headquarters, military officials said. The building was not struck, but the police chief, Col. Jalil Nahi Hassoun, and 11 other policemen were killed as they fought off Al Qaeda linked gunmen outside.

At least five Al Qaeda fighters were killed, said a U.S. military official who was not authorized to release the new details and spoke on condition of anonymity.

U.S. paratroopers from the 82nd Airborne Division came under small arms and rocket-propelled grenade fire when they rushed to the scene, the military said in a separate statement. Two Americans were wounded and a vehicle was damaged.

Samarra was the scene of the Feb. 22, 2006, bombing that destroyed a major Shiite shrine and triggered the wave of Sunni-Shiite reprisal attacks that has plunged this country into civil conflict. U.S. and Iraqi officials blame that bombing on Al Qaeda, which has been active in the city for years.

The deadliest attack Sunday against U.S. forces occurred in Diyala, where six U.S. soldiers and a Russian photojournalist were killed when a massive bomb destroyed their vehicle, the U.S. military and the editor of the Russian edition of Newsweek magazine said. Two U.S. soldiers were wounded, the military said.

The U.S. military confirmed Monday that an American brigade commander was wounded by gunfire while inspecting a security barrier being built to separate a Sunni enclave from Shiite areas in Baghdad.

Construction of the wall has drawn strong criticism from residents and Sunni clerics who say it is a form of sectarian discrimination, and much of the work is done at night. But Farris was wounded while conducting a daytime survey. The attack was witnessed by an AP Television News cameraman, but the military declined to identify the commander, Col. B.D. Farris, until Monday.

The international Red Cross on Monday said it would scale up its operations in Iraq to provide food, water and medical treatment for hundreds of thousands of people struggling to survive amid the security situation in the country.

"This conflict is inflicting immense suffering on all Iraqis," said Beatrice Megevand-Roggo, head of Middle East operations for the International Committee of the Red Cross. "People directly affected by the crisis are finding it increasingly difficult to cope."

The Geneva-based humanitarian organization said it was seeking $28.9 million, bringing its total 2007 budget for Iraq to more than $75 million.

Complete coverage is available in FOXNews.com's Iraq Center.