BAGHDAD — A suicide bomber drove a truck rigged with explosives into a checkpoint manned by members of a U.S.-backed security volunteer group in a town north of Baghdad on Monday, killing at least 12 people, police and a member of the volunteer group said.

Another three people were missing following the explosion in the town of Mishada, 20 miles north of the capital, on part of the main highway linking Baghdad to northern Iraq, according to a police officer and Adil al-Mishhadani, a member of the volunteer group.

The groups, known as Awakening Councils and dubbed Concerned Local Citizens by the U.S. military, have been credited with helping reduce violence in the country. The groups are made up of mainly Sunni tribal fighters who turned against al-Qaida in Iraq earlier this year and are now paid by the U.S. military to help provide security.

But they're also increasingly becoming targets in Iraq.

All 12 people killed in Monday's attack, as well as the three people missing, were believed to be council members, al-Mishhadani said.

In a separate attack, a female suicide bomber detonated herself near a police patrol, wounding five policemen and four civilians in the town of Baqouba, 35 miles northeast of Baghdad, said police Brigadier Khudeir al-Tamimi.

Last week, a new audiotape of al-Qaida leader Osama bin Laden emerged warning Iraq's Sunni Arabs against joining the Awakening Councils or participating in any unity government.

He denounced Abdul-Sattar Abu Risha, the leader of the Awakening Council in Anbar province, where the movement started. Abu Risha was killed in a bombing in September.

The Awakening Council movement now includes more than 70,000 fighters in Anbar, Baghdad and other Sunni-dominated provinces. The councils, along with a surge of extra U.S. troops into Baghdad and a cease-fire declared by radical Shiite extremist Muqtada al-Sadr for his Mahdi Army militia, are credited with a 60 percent reduction in overall violence in the country since June.

In the capital, a mortar round wounded three civilians when it landed on a house in Baghdad's western neighborhood of Amariyah Monday, a Baghdad police officer said on condition of anonymity because he was not authorized to release information to the media. The intended target was believed to have been a nearby Awakening Council headquarters.

To the south in Wasit province, gunmen shot and wounded an Awakening Council member in the village of al-Hafriyah, 100 miles southeast of Baghdad, police said.

In the town of Khalis, 50 miles north of Baghdad, gunmen traded fire with police and Awakening Council members, leaving one council member and one policeman dead and wounding three policemen, a police officer said.

Separately, six mortar rounds landed near an Iraqi Army checkpoint near the town, wounding two soldiers, the officer said.

In other violence Monday, a roadside bomb targeting a patrol near the Iranian border killed two Iraqi soldiers and injured another four, police said.

The attack occurred shortly after dawn in the frontier town of Mandali, about 30 miles from the Iranian border in Diyala province northeast of Baghdad, a police officer said on condition of anonymity as he was not authorized to release information to the media.

Diyala has become one of Iraq's most dangerous areas. Most of the activities of al-Qaida in Iraq have been pushed out Anbar province and Baghdad, into Diyala and the northern city of Mosul, following the surge of U.S. troops and the work of Awakening Councils in Anbar.

"Diyala has been one of the tougher fights," U.S. Navy Rear Adm. Gregory Smith said in Baghdad Sunday. "Diyala is an area of significant interest for al-Qaida."

"We realize that security in Iraq is very fragile and tenuous," Smith said. Although much progress had been made, "there is no place in Iraq today that is safe from terrorism."