Nearly 6,000 Sunni Arab residents joined a security pact with US forces in what US officers described as a critical step in plugging the remaining escape routes for extremists flushed from former strongholds.
The new alliance -- called the largest single volunteer mobilization since the war began -- covers the "last gateway" for groups such as al-Qaeda in Iraq seeking new havens in northern Iraq, US military officials said on Wednesday.
US commanders have tried to build a ring around insurgents who fled military offensives launched earlier this year in the western Anbar Province and later into Baghdad and surrounding areas. In many places, the US-led battles were given key help from tribal militias -- mainly Sunnis -- that had turned against al-Qaeda and other groups.
Extremists have sought new footholds in northern areas once loyal to Saddam Hussein's Baath party as the US-led gains have mounted across central regions. But their ability to strike near the capital remains.
A woman wearing an explosive-rigged belt blew herself up near a US patrol near Baqubah, about 56km northeast of Baghdad, the military announced on Wednesday. The blast on Tuesday -- a rare attack by a female suicide bomber -- wounded seven US troops and five Iraqis, the statement said.
The ceremony to pledge the 6,000 new fighters was presided over by a dozen sheiks -- each draped in black robes trimmed with gold braiding -- who signed the contract on behalf of tribesmen at a small US outpost in north-central Iraq.
For about US$275 a month -- nearly the salary for the typical Iraqi policeman -- the tribesmen will man about 200 security checkpoints beginning Dec. 7, supplementing hundreds of Iraqi forces already in the area.
About 77,000 Iraqis nationwide, mostly Sunnis, have broken with the insurgents and joined US-backed self-defense groups.
Those groups have played a major role in the lull in violence: 648 Iraqi civilians have been killed or found dead in November to date, according to figures compiled by the Associated Press. This compares with 2,155 in May as the so-called "surge" of nearly 30,000 additional troops gained momentum.
US troop deaths in Iraq have also dropped sharply. So far this month, the military has reported 35 deaths -- including a US soldier killed on Wednesday in western Baghdad -- compared with 38 last month. In June, 101 US soldiers died in Iraq.
Village mayors and others who signed Wednesday's agreement say about 200 militants have sought refuge in the area, about 50km southwest of Kirkuk on the edge of the semi-autonomous Kurdish region. Hawija is a predominantly Sunni Arab cluster of villages that has long been a flashpoint.
The recently arrived militants have waged a campaign of killing and intimidation to try to establish a new base, said Sheikh Khalaf Ali Issa, mayor of Zaab village.
"They killed 476 of my citizens and I will not let them continue their killing," Issa said.
With the help of the new Sunni allies, "the Hawija area will be an obstacle to militants, rather than a pathway for them," said Major Sean Wilson, with the Army's 1st Brigade, 10th Mountain Division. "They're another set of eyes that we needed in this critical area."