Eighteen young women purportedly belonging to a suicide bombing network in northern Iraq surrendered to U.S. forces on Wednesday, a top U.S. commander said.

Maj. Gen. Mark Hertling, who leads U.S. forces in northern Iraq, said the women turned themselves in after a local cleric and their families persuaded them that suicide attacks violated the tenets of Islam.

The number of bombings carried out by women has spiked this year even as overall violence has declined, and Hertling said Wednesday's surrender would deal a significant blow to recruitment efforts.

U.S. military figures show 31 attacks by 35 female suicide bombers so far this year, compared with eight in 2007.

Hertling said the cleric approached U.S. forces earlier this week to tell them about a group of 21 women who were part of a suicide bombing network. On Wednesday, 18 of the women surrendered and signed a form promising not to conduct attacks as part of a reconciliation program, Hertling said.

"This will significantly affect at least part of the female network cells in the northern area of operations," he told The Associated Press in a telephone interview.

He said the women were young and from prominent families headed by sheiks, but he refused to give other details, including their ages or locations, citing ongoing interrogations and the need to protect the cleric.

Hertling also declined to say what would happen to the women but said they were currently being confined together while the case was investigated and preliminary indications were that Al Qaeda in Iraq was to blame.

The use of female suicide bombers is part of a shift in insurgent tactics to avoid detection at U.S.-Iraqi military checkpoints that have become ubiquitous in Iraq as part of increased security measures.

Iraqi women often are allowed to pass through male-guarded checkpoints without being searched, and they traditionally wear flowing black robes that make it easier to hide explosives belts.

To counter the threat, the U.S. military has stepped up efforts to recruit women for the Iraqi security forces.

Hertling said 17 female suicide bombers had struck northern Iraq in the 15 months since his unit arrived, killing 79 people and wounding 183, but he welcomed a recent downturn with only one attack this month and one in October.

The main targets were Iraqi police and civilians in public places, he said.

Some female bombers appear motivated by revenge, like the woman who killed 15 people in Diyala province on Dec. 7. She was a former member of Saddam Hussein's Baath party whose two sons joined Al Qaeda in Iraq and were killed by Iraqi security forces.

But activists and U.S. commanders also believe Al Qaeda in Iraq is increasingly seeking to exploit women who are unable to deal with the grief of losing husbands, children and others to the violence.