* Security forces worried al Qaeda to rebase in Anbar
* Attack comes month after similar attack in provincial capital (Adds analyst quote, details)
By Fadhel al-Badrani
FALLUJAH, Iraq, March 5 (Reuters) - Gunmen disguised as police raided checkpoints and homes in western Iraq on Monday, killing at least 27 members of the security forces, police said, in an attack the authorities said bore the hallmarks of al Qaeda.
The attacks in Anbar, once Iraq's most violent province, raise concern that Iraq's branch of al Qaeda may regain a foothold there after the withdrawal of U.S. troops in December.
Anbar was almost entirely under control of al Qaeda during the height of Iraq's insurgency from 2005-07, when the militants were defeated by local tribesmen and U.S. forces.
Mohammed Fathi, spokesman for the governor of Anbar province, said the latest attack bore the "fingerprints of al Qaeda".
A police source, who had been ferrying victims to the hospital morgue, said gunmen dressed in uniforms of the security forces had driven from checkpoint to checkpoint slaughtering police in Haditha, a town 190 km (120 miles) northwest of Baghdad.
"The gunmen used security vehicles and from 2:00 a.m. (2300 GMT) until 3:30 a.m. they carried out attacks on checkpoints in central Haditha and the nearby town of Barwana," the police source, who did not give his name because he was not authorised to speak to the media, told Reuters in Fallujah.
Fathi, the governor's spokesman, said the attackers arrived at checkpoints with fake arrest warrants, confiscated the mobile phones of the police guards and executed them.
Another police source said one of the damaged vehicles left behind had fliers signed by an al Qaeda affiliate, the Islamic State of Iraq, which threatened Iraqi police forces with death if they did not abandon their jobs.
The 27 dead included a lieutenant colonel and a captain who were dragged out of their homes in Haditha and killed, the police source said. A curfew was imposed on the town and its exits were sealed off.
One gunman was killed in the attacks, the source said. Three policemen survived the attacks with wounds and were being treated at Haditha hospital.
A medical source at Hadita hospital confirmed the hospital had received 27 bodies of slain victims and was treating three wounded.
Fathi said the attackers may have intended to derail a summit of Arab leaders set for later this month. Iraq is due to host a summit of the Arab League for the first time since the invasion that toppled Saddam Hussein, and its leaders have been at pains to say security is under control.
Al Qaeda and other Sunni militant groups oppose the Shi'ite-led government in Baghdad and say they will continue to fight despite the withdrawal of U.S. forces last year.
They have continued to strike mainly against the security forces. Coordinated early morning attacks that mainly targeted police in Shi'ite areas killed at least 60 people across the country on Feb. 23.
In Anbar in January, 10 people were killed when gunmen wearing explosive belts stormed a police building in provincial capital Ramadi.
Iraqi political analyst Ibrahim al-Suamidaie blamed the decentralised security structure in the country for the ease with which gunmen are able to exploit weaknesses in the police forces.
"The fact that the security portfolio is not centralised is the biggest reason. This is not just a breach, it's a catastrophe," he told Reuters.
Once an al Qaeda stronghold and Iraq's most violent province in 2004-06, Anbar was subdued in 2006-07 when tribal leaders and former insurgents turned against the fighters. Tribal "awakening" militia joined U.S. forces and forced al Qaeda out.
Tension has risen between Anbar and the Shi'ite-led central government in recent months, following an arrest campaign against former members of Saddam's banned Baath party. (Additional reporting by Aseel Kami, Suadad al-Salhy and Yara Bayoumy in Baghdad; Writing by Yara Bayoumy and Peter Graff Editing by Maria Golovnina)