The Iraqi army shelled the western city of Falluja with mortar bombs overnight to try to wrest back control from Sunni Muslim militants and tribesmen, killing at least eight people, tribal leaders and officials said on Saturday.

Falluja has been held since Monday by militants linked to Al-Qaida and by tribal fighters united in their opposition to Iraqi Prime Minister Nuri al-Maliki, in a serious challenge to the authority of his Shi'ite-led government in Anbar province.

Medical sources in Falluja said another 30 people were wounded in shelling by the army.

Al-Qaida's Islamic State of Iraq and the Levant (ISIL) has been tightening its grip in the Sunni-dominated desert province, near the Syrian border, in recent months in a bid to create an Islamic state across the Iraqi-Syrian borders.

In Ramadi, the other main city in Anbar, tribesmen and the army have worked together to counter Al-Qaida militants seeking to take control.

But in Falluja, ISIL's task has been made easier by the cooperation of tribesmen, who have joined forces with it against the government.

Tension has been running high in Anbar - once the heart of Iraq's insurgency after the 2003 U.S.-led invasion - since Iraqi police broke up a Sunni protest camp on Monday. At least 13 people were killed in those clashes.

The escalating tension shows the civil war in Syria, where mostly Sunni rebels are battling Syrian President Bashar Assad, who is backed by Shi'ite 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.

Militants have deployed snipers on top of the empty houses and government buildings to prevent the army from entering the city.