Iraq forces launch assault on IS bastion Fallujah
An image grab taken from a video released on March 17, 2014 by the Islamic State of Iraq and the Levant's al-Furqan Media allegedly shows ISIL fighters raising their weapons with the Jihadist flag at an undisclosed location (AFP Photo)

Iraqi forces have launched a major assault to retake Fallujah, the scene of deadly battles during the US occupation and one of the toughest targets yet in Baghdad's war on the Islamic State group.

Prime Minister Haider al-Abadi, commander-in-chief of the armed forces, made the announcement early Monday and then visited the battle's operations room.

"We are beginning the operation to liberate Fallujah," he said in a statement. "The Iraqi flag will be raised high over the land of Fallujah."

Abadi said that special forces, soldiers, police, militia forces and pro-government tribesmen were taking part in the offensive to retake the city, located in Anbar province just 50 kilometres (30 miles) west of Baghdad.

Iraqi forces had not yet entered the city, but an AFP photographer near Fallujah said they were advancing while aircraft carried out strikes on targets inside the city.

Abadi's announcement settled the issue of which IS-heldcity Iraq should seek to retake next -- a subject of debate among Iraqi officials and international forces helping the country fight the jihadists.

Iraq's second city Mosul was the US military's recommended target, but powerful Iraqi militias may have helped force the issue by deploying reinforcements to the Fallujah area in preparation for an assault.

Iraqi forces have begun laying the groundwork for the recapture of Mosul, but progress has been slow and an assault to retake the city remains far off.

- Thousands of civilians -

The US-led anti-IS coalition carried out seven strikes in the Fallujah area last week, and Iraq said it also bombed the city with US-supplied F-16 warplanes.

On Sunday, Iraq's Joint Operations Command warned civilians still in Fallujah -- estimated to number in the tens of thousands -- to leave the city.

It said that families who cannot leave should raise a white flag over their location and stay away from IS headquarters and gatherings.

Officials said several dozen families had fled the city, but IS has sought to prevent civilians from leaving, and forces surrounding Fallujah have also been accused of preventing foodstuffs from entering.

Iraqi forces have in recent days been massing around the city, which has been out of government control since January 2014.

Anti-government fighters seized it after the army was withdrawn, and Fallujah later became one of IS's main strongholds.

Fallujah and Mosul, the capital of the northern province of Nineveh, are the last two major cities IS holds in Iraq.

Fallujah is almost completely surrounded by Iraqi forces, which have regained significant ground in Anbar province in recent months, including its capital Ramadi further up the Euphrates River valley.

- Insurgent bastion -

American forces launched two major assaults on Fallujah in 2004 in which they saw some of their heaviest fighting since the Vietnam War.

Iraqi forces would have the advantage of greater knowledge of the area, especially if they employ pro-government Anbar tribal fighters in the battle.

But they lack the training and enormous firepower that American forces brought to the Fallujah battles.

IS overran large areas north and west of Baghdad in June 2014, and Iraqi forces performed dismally during the initial offensive despite significantly outnumbering thejihadists.

But the "caliphate" the jihadist group subsequently proclaimed has been shrinking lately, with anti-IS forces scoring major victories in both Iraq and Syria, where the group has also seized significant territory.

Several of its key leaders, including the group's number two, have been killed in air strikes by the US-led coalition.

The coalition earlier this month announced it had killed an IS fighter described as the group's top military leader in Anbar.

In an audio message released on Saturday, IS spokesman Abu Mohammed al-Adnani appeared to acknowledge that the group was likely to lose more ground.

But the battle for Fallujah -- a city that has long been a Sunni insurgent stronghold and which IS has had some two years to reinforce -- will be one of the toughest challenges they have yet faced.