Iraqi forces raised the national flag over the main government compound in Fallujah on Friday, top commanders said, a breakthrough in the nearly four-week-old offensive against the Islamic State group's bastion.

They met limited resistance from IS fighters, who were fleeing the city, the commanders told AFP, leaving the organisation on the brink of losing one of the most emblematic strongholds in its two-year-old "caliphate".

It is the latest setback for the jihadists who have also lost territory in neighbouring Syria and in Libya in recent weeks, although US Central Intelligence Agency director John Brennan warned on Thursday that they remain a formidable force with global reach.

"The counter-terrorism service and the rapid response forces have retaken the government compound in the centre of Fallujah," the operation's overall commander, Lieutenant General Abdulwahab al-Saadi, told AFP.

Raed Shaker Jawdat, Iraq's federal police chief, confirmed the advance.

"The liberation of the government compound, which is the main landmark in the city, symbolises the restoration of the state's authority" in Fallujah, he said.

Saadi said the Iraqi flag was raised above government buildings in the compound and claimed that "Iraqi forces have now liberated 70 percent of the city."

In the deserted, recently reconquered neighbourhoods of the insurgent bastion known in Iraq as the "City of Mosques", elite forces were consolidating positions, stocking up on food and weapons.

Dozens of bodies of dead IS fighters were left to rot under blankets amid the rubble of homes destroyed by air strikes, rockets or controlled explosions of the hundreds of bombs the jihadists themselves laid across the city.

- 'Little resistance' -

The government lost control of Fallujah in 2014, months before IS took second city Mosul and swept across large parts of the country.

Fallujah, just 50 kilometres (30 miles) west of Baghdad, is one of IS's key historical bastions and its loss would leave Mosul as the only major Iraqi city under its control.

The US-led coalition, which has carried out air strikes in support of the Fallujah operation, had initially favoured focusing efforts on recapturing Mosul.

Prime Minister Haider al-Abadi, who was facing huge political pressure over the reform of his own government when he declared the launch of the Fallujah operation, has vowed to defeat IS nationwide by the end of the year.

In the hours before the latest push into the heart of Fallujah, Iraqi forces retook several neighbourhoods in quick succession.

"This operation was done with little resistance from Daesh," Saadi said, using an Arabic acronym for IS.

"They know, by experience, that a small number of IS fighters in urban terrain cannot stop an Iraqi security forces ground assault supported by coalition airstrikes," Patrick Martin, Iraq analyst at the Institute for the Study of War, said.

After months of military operations aimed at completely sealing off the city, IS had been expected to fight to the death in a protracted suicide holdout, but recent developments suggest the siege was porous.

"There is a mass flight of Daesh to the west that explains this lack of resistance. There are only pockets of them left and we are hunting them down," Saadi said.

Tens of thousands of civilians have been forced from their homes since the operation began last month.

- Aid groups overwhelmed -

The first to escape IS rule were in rural outlying areas, in the early phase of the operation which saw a myriad different Iraqi forces seal the siege of the city.

Residents of the city centre had been trapped in dire conditions for days, but recent advances have allowed large numbers to escape.

IS "likely quickly discovered that they did not have the forces available to exert social control over the city and prevent civilians from fleeing once the assault into central Fallujah began," Martin said.

The Norwegian Refugee Council, which runs camps for the displaced near Fallujah, said the sudden influx meant relief was drying up fast.

"Thousands of civilians from Fallujah are right now heading towards displacement camps in a dramatic development that is overwhelming emergency aid provision and services," it said.

With IS on the retreat in the city, a window has opened for civilians to leave but the journey remains dangerous, with several cases of fleeing civilians killed or wounded by roadside bombs.

There were an estimated 50,000 people in the city when the operation began but it is unclear how many remain.

Civilians have been used as human shields by IS, and those who managed to flee face the risk of sectarian-motivated abuse by elements of the pro-government forces.

Fallujah is a Sunni Muslim city, and the involvement of Shiite militia groups in the operation had raised fears of sectarian revenge attacks.

Iraq forces eye Mosul after Fallujah breakthrough: minister
Kirkuk, Iraq (AFP) June 18, 2016 - Iraqi forces renewed their offensive against the Islamic State group around second city Mosul on Saturday after driving the jihadists out of most of Fallujah, the defence minister said.

Mosul is the last major urban centre in Iraq still under IS control after Iraqi forces raised the national flag over government headquarters in the heart of Fallujah on Friday.

Iraqi commanders announced the launch of an offensive to retake Mosul and surrounding Nineveh province in March but under domestic political pressure the government diverted its forces to Fallujah, just west of Baghdad, last month.

"We started at 5:00 am (0200 GMT) the second phase of the liberation of Nineveh," Defence Minister Khaled al-Obeidi told AFP.

"The target of the operation is to take Qayyarah and make it a launchpad for Mosul," Obeidi said.

Qayyarah, which has an airfield, lies across the River Tigris from the main base for pro-government forces in the Kurdish-controlled area of Makhmur.

It is some 60 kilometres (35 miles) south of Mosul.

The offensive, which has been repeatedly pushed by Washington, has support from US-led forces, notably in the shape of a US Marine artillery post outside Makhmur.

On Friday, Prime Minister Haider al-Abadi promised that the liberation of Mosul was "very near" as he declared victory in the four-week offensive to retake Fallujah.

Abadi said that only small pockets of IS resistance remained to be cleared from the jihadists' emblematic bastion.

But IS still firmly controls northern neighbourhoods of Fallujah where it is believed to be holding thousands of civilians as human shields.

IS attack in Iraq kills 15 members of security forces
Kirkuk, Iraq (AFP) June 17, 2016 - The Islamic State group has killed 15 members of the local security forces in attacks on villages in northern Iraq, officials said on Friday.

The attack was launched late Thursday on villages east of the restive town of Tuz Khurmatu, which lies in an ethnically mixed area 160 kilometres (100 miles) north of Baghdad.

The ensuing clashes killed 15 members of the police, of a Turkmen paramilitary organisation and of the Kurdish peshmerga.

"Twelve people, six members of the police and six of the Hashed al-Turkmani were killed in the fighting against Daesh (IS)," said Abu Ridha al-Najjar, the local leader of the Turkmen group.

"The attack began in Birahmed area last night," he said of a cluster of villages west of Tuz Khurmatu.

The Hashed al-Turkmani is a local branch of the Hashed al-Shaabi, a national umbrella organisation dominated by Tehran-backed Shiite militias.

In and around Tuz Khurmatu and other areas in Iraq's Turkmen heartland, Shiite Turkmen fighters belonging to these militias fight under the banner of the Hashed al-Turkmani.

Mullah Karim Shakur, a local peshmerga official, said three Kurdish fighters were also killed in the fighting.

The attack was claimed by IS in an online statement. The jihadists still hold nearby areas, such as the town of Hawijah and parts of the Hamreen mountain range.

Tuz Khurmatu is in an area theoretically under Baghdad's authority, but the autonomous northern region of Kurdistan has a military presence there and claims the area as its own.

The town has been the scene of deadly skirmishes between Shiite Turkmen and Kurdish forces in recent months.