Iraqi police and tribal fighters claimed to have retaken the western city of Falluja from al-Qaida affiliates on Saturday after heavy clashes that left scores of dead there and in the neighbouring city of Ramadi in recent days.

Amid reports of civilians fleeing fighting elsewhere in Anbar and Ninevah provinces, social media in the country described continuing serious violence in western Iraq between Sunni Islamist militants and forces of the country's Shia-dominated government.

Officials and witnesses said the northern and eastern parts of Falluja were still under the control of tribesmen and militants on Saturday despite shelling from government troops, Reuters reported.

The sudden escalation of violence broke out early last week after a government clampdown on a year-long protest camp in Ramadi demanding more civil rights for the country's Sunni minority.

Falluja has been held since Monday by militants from the Islamic State of Iraq and the Levant (ISIS), which is also fighting in neighbouring Syria, in the most serious challenge yet to the authority of the Shia-led government in Anbar province.

According to a local journalist working for CNN in Falluja, tribal forces and police were again in control of a city where US forces and militants fought one of the bloodiest battles of the Iraq war. Other reports suggested, however, that ISIS fighters still controlled parts of Falluja.

The claims, none of which could immediately be confirmed, follow a night that saw government forces mortar al-Qaida positions in the north and east of the town.

The escalating tension shows that the civil war in Syria, where mostly Sunni rebels are battling President Bashar al-Assad, who is backed by Shia power Iran, is spilling over to other countries like Iraq threatening its delicate sectarian balance.

Officials and witnesses in Falluja said the northern and eastern parts of the city were under the control of tribesmen and militants after residents fled the neighbourhoods to take refuge from the army shelling. The fighting in Falluja came as al-Qaida affiliates across Iraq, Syria and Lebanon appeared to suffer a day of reverses on Saturday amid escalating conflicts between al-Qaida-backed groups and more moderate Sunni fighters.

Syria, meanwhile, saw continued fighting between ISIS and the Free Syrian Army not least in Idlib province where it was claimed Al Qaeda fighters had withdrawn from several locations after heavy clashes.

In a separate development it was announced in Lebanon that the suspected leader of an alQaida-linked group that claimed responsibility for bombing the Iranian embassy in Beirut two months ago had died in custody on Saturday, security sources said.

Majid bin Mohammad al-Majid, a Saudi national wanted by authorities in his own country, had been suffering from kidney failure and went into a coma on Friday, the sources said. He died in a military hospital in Beirut, they added.

Majid was believed to be the leader of the Abdullah Azzam Brigades, which have claimed attacks across the region, most recently the double suicide assault on Iran's Beirut embassy, which killed at least 25 people.