BAGHDAD - Three suspected al Qaeda militants, including two sisters, beheaded their uncle and his wife, forcing the couple's children to watch, Iraqi police said on Friday.

The militants considered that school guard Youssef al-Hayali was an infidel because he did not pray and wore western-style trousers, they told police interrogators after being arrested in Diyala province northwest of Baghdad.

The three cousins executed Hayali and his wife Zeinab Kamel at the all-boys school in Jalawlah in Diyala province, village police chief Captain Ahmed Khalifa said.

No further details were available.

Sunni Arab communities across Iraq have been turning against al Qaeda because of its indiscriminate killings and strict interpretation of Islam, which includes a ban on smoking in public and forcing schoolgirls to wear veils.

Sunni Arab tribal sheikhs have been organising their young men into neighbourhood police units to drive out al Qaeda, a practice which U.S. and Iraqi officials say has helped bring down violence levels across Iraq.

Security operations by U.S. and Iraqi troops have also been targeting al Qaeda in ethnically and religiously mixed Diyala in recent months after the Sunni Islamist fighters were driven out of western Anbar province.

The U.S. military said earlier in November that it was sending 3,000 soldiers home from the province but added that the overall number of forces in the province would not decrease.

The United States poured an extra 30,000 troops into Iraq from mid-February in a bid to stop the country from sliding into sectarian civil war.

U.S. and Iraqi officials say the troop "surge" and more efficient Iraqi security forces have also helped bring about sharp falls in U.S. military and Iraqi civilian casualties in the past two months.