Iraqi officials say gunmen killed 22 Shi'ite Muslim pilgrims as they traveled through Iraq's western desert on a trip to Syria.

Officials said Tuesday the gunmen stopped a bus of pilgrims at a fake security checkpoint in Anbar province on Monday. They asked the women and children to get off the bus and killed 22 Shi'ite Muslim men.

The pilgrims had planned to travel from Iraq's southern Shi'ite city of Karbala to visit a Shi'ite shrine in Iraq's northern neighbor. It is unclear if the group was on their way to Syria or returning to Karbala when the gunmen attacked.

Shi'ite pilgrims are a frequent target of Sunni insurgents in Iraq. Bomb blasts in Karbala killed Shi'ite pilgrims in July as thousands gathered for a religious festival in the city.

Monday's killings took place as the U.S. moves ahead with plans to remove its forces from Iraq by the end of the year. Both U.S. and Iraqi officials have expressed concern about Baghdad's ability to cope with security after the pull-out.