HAWIJA, Iraq - Nearly 6,000 Sunni Arab residents joined a security pact with American forces yesterday in what U.S. officers described as a critical step in plugging the remaining escape routes from former extremist strongholds.

The new alliance - called the single largest volunteer mobilization since the war began - covers the "last gateway" for groups such as al-Qaeda in Iraq seeking new havens in northern Iraq, U.S. military officials said.

U.S. commanders have tried to build a ring around insurgents who fled military offensives launched this year in western Anbar province and later in Baghdad and surrounding areas. In many places, the U.S.-led battles were given key help from tribal militias - mainly Sunnis - that had turned against al-Qaeda in Iraq and other groups.

Extremists have sought new footholds in northern areas once loyal to Saddam Hussein's party as the U.S.-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 an American patrol near Baqubah, about 35 miles northeast of Baghdad, the military said yesterday. The blast Tuesday - a rare attack by a female suicide bomber - wounded seven U.S. troops and five Iraqis, the statement said.

The ceremony to pledge the 6,000 new fighters was presided over by dozen sheikhs who signed the contract on behalf of tribesmen at a small U.S. outpost in north-central Iraq.

For about $275 a month, nearly the salary for the typical Iraqi policeman, the tribesmen will man about 200 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 U.S.-backed self-defense groups, known variously as volunteers, concerned local citizens, or members of awakening councils.

Those groups have played a role in the lull in violence.

Village mayors and others who signed yesterday's pact say about 200 militants have sought refuge in the area, 30 miles southwest of Kirkuk on the edge of north Iraq's semiautonomous Kurdish region. Hawija is a mainly Sunni Arab cluster of villages that has long been an insurgent flash point.

The recently arrived fighters have waged a campaign of killing and intimidation to try to establish a new base, said Sheikh Khalaf Ali Issa, mayor of Zaab village.

With the help of the new Sunni allies, "the Hawija area will be an obstacle to militants, rather than a pathway for them," said Maj. Sean Wilson.

Refugees Returning to Iraq

About 20 buses

carrying hundreds of Iraqi refugees returned to Baghdad from Syria yesterday, a move Iraqi leaders hailed.

The convoy

was the first from an Iraqi-funded effort to speed the return of families who fled violence. Prime Minister Nouri al-Maliki was to give each returning family $750 to help rebuild their lives.

Officials gave

varying figures for the number who returned. Envoys in Syria said 800 would leave; an Iraqi official put the figure at more than 400.

Thousands of Iraqis

in Syria have headed home in past weeks, but Iraqi leaders aim to accelerate the flow by offering to pay for trips.

The program also seeks

to win favor from countries such as Syria and Jordan, struggling with roughly

2.2 million refugees. Syria tightened visa rules for Iraqis in hopes of forcing people to go home and blocking new refugees.

- Associated Press