At least 21 people were killed in Iraq Sunday morning in a series of attacks on Iraqi police stations. The attack was part of escalating violence as U.S. and Iraqi forces prepare a major assault of the insurgent stronghold of Fallujah.

Militants attacked police stations in the towns of Haditha and Haqlaniyah, in the north western Anbar region. In Haqlaniyah, the attackers reportedly captured and executed a group of Iraqi policemen, including the head of security police for the area, Shahir al-Ghureifi.

On Saturday, four car bombs in the city of Samarra killed at least 33 people, many of them Iraqi police. Also on Saturday, 16 U.S. Marines were wounded in Ramadi.

The escalating attacks across the northwestern region of Iraq appear to be in response to the U.S. and Iraqi siege of the insurgent stronghold of Fallujah.

About 12,000 U.S. and Iraqi forces have surrounded Fallujah, awaiting Interim Iraqi Prime Minister Iyad Allawi's order to attack. U.S. war planes have conducted air strikes targeting alleged rebel arms caches in the city. U.S. Marines pounded the rebel enclave with artillery fire from Saturday night through Sunday morning. Many of the city's civilians have fled.

The attack on Fallujah aims at regaining control of the area and weakening the insurgency ahead of planned Iraqi elections in January. Mr. Allawi demands the surrender of Iraqi and foreign fighters who reportedly use the city as their base of operations.