Al-Qaeda's front group in Iraq said on Friday that it carried out a shooting spree in western Iraq this month that left 27 police dead, including two officers killed execution-style.

The Islamic State of Iraq (ISI) said 90 fighters carried out the pre-dawn attack in the town of Haditha on March 5, and denied official claims of security forces having arrested insurgents behind the assault.

"A group of heroes of the Islamic State launched a foray targeting one of the cities of Anbar which is full of apostates," an ISI statement posted on jihadist forum Honein said, referring to Anbar province where Haditha lies.

"Ninety fighters pledged for death, many of them wore explosives belts, and took off in four convoys towards Haditha," said the statement, signed by ISI spokesman Abu Mohammed al-Adnani.

It said the attackers killed 40 police in all, whereas security and medical officials have said 27 died. The ISI also claimed it suffered no casualties at the hands of security forces, with just two fatalities occurring as a result of friendly fire.

The March 5 attack saw insurgents dressed in military uniforms simultaneously attacking two checkpoints in the east and west of Haditha before storming other security posts and raiding the homes of two officers.

They killed bodyguards at each of the officers' houses and kidnapped both. Their bodies were later found in a marketplace and an alleyway respectively.

Iraqi authorities said the following day that they arrested at least 13 insurgents who allegedly took part in the attack, but the ISI said none of the group who launched the assault were detained.

The violence came ahead of a March 29 Arab League summit in Baghdad, the first meeting of the 22-nation bloc to be held in the Iraqi capital since now-executed dictator Saddam Hussein's 1990 invasion of Kuwait.

Haditha was one of several towns along the Euphrates valley that became Al-Qaeda strongholds after the US-led invasion ousted Saddam in 2003.

However, from late 2006, local Sunni Arab tribes sided with the US military and unrest dwindled in Anbar as rebel fighters were ejected from the region.