A homicide bomber on a motorcycle struck a checkpoint manned by Iraqi police and U.S.-allied Sunni fighters Monday north of Baghdad, killing four people, officials said.

The blast occurred about 200 yards away from the house of the head of the local awakening group, which has joined forces with the Americans against Al Qaeda in Iraq in Tarmiyah, according to a police official and a member of the group.

Those killed included a policeman, two awakening council guards and a civilian, according to the police. The officials spoke on condition of anonymity because they weren't authorized to release the information.

A U.S. soldier also was killed and two others wounded Monday in a roadside bombing in Salahuddin province, raising to at least 4,082 the number of American service members who have died in Iraq since the war started in March 2003.

Salahuddin is a predominantly Sunni province that includes Tarmiyah, Saddam Hussein's hometown of Tikrit and other cities.

Another roadside bomb exploded near an Iraqi army checkpoint on the road that leads to the Baghdad International Airport, wounding five people, including one Iraqi soldier and four civilians, police said.

The blast sent up a huge plume of black smoke and caused vendors at nearby kiosks selling soft drinks to run for cover.

The attacks came a day after the U.S. military said violence in Iraq had reached its lowest levels in four years.

Rear Adm. Patrick Driscoll, a U.S. military spokesman, said Sunday that the number of attacks in the past week decreased to a level "not seen since March 2004," although he did not give specific figures.

He also warned that Al Qaeda in Iraq was "off-balance and on the run" but remains a very lethal threat, tempering remarks by U.S. Ambassador Ryan Crocker a day earlier that the terror network was closer than ever to being defeated.

Despite recent security gains, violence has been slower to decline in northern Iraq after insurgents fled to the area to escape U.S.-led crackdowns in Baghdad and surrounding areas.

Many of the attacks have targeted the so-called awakening councils, which have been a key factor in the drop in violence.

Suspected Al Qaeda fighters also kidnapped Sheik Saleh al-Karkhi and his brother after blowing up his house in the village of Busaleh in the volatile Diyala province north of the capital, a police official said, declining to be identified because he wasn't supposed to release the information.

The official, who read the report at the provincial military operations command center in Baqouba, said al-Karkhi was probably abducted because he had set up two awakening councils in the area and "took it upon himself to fight Al Qaeda."

The U.S. military has consistently been cautious about recent security gains amid fears that Al Qaeda and other insurgents are trying to regroup after suffering setbacks from military operations as well as a Sunni revolt against the terror network.

But officials have been taking a more confident tone over security gains in Iraq in recent weeks, particularly since the high profile crackdowns in Mosul and Sadr City, and the southern city of Basra. Those sweeps aim to impose Iraqi government control in areas that have been under the control of Shiite militias or Sunni insurgents.