BAGHDAD: Iraqi and US-led coalition aircraft decimated the militant Islamic State group's forces fleeing the Fallujah area, destroying hundreds of vehicles and killing dozens of militants, officials said on Thursday.

Fallujah, a city west of Baghdad, was wrested from IS by Iraqi forces on Sunday after more than two years under the group's control.

The strikes -- which the Iraqi government said took place from Wednesday to Thursday -- compounded what was already a major defeat for the militants.

The Pentagon estimated that coalition strikes destroyed some 175 IS vehicles, while Iraq's Joint Operations Command said the country's forces destroyed 603.

Those figures could not be independently confirmed.

“Over the last two days, the Iraqi security forces and the coalition conducted strikes against two large concentrations of (IS) vehicles and fighters,” Pentagon spokesman Matthew Allen said.

Allen said the coalition destroyed an estimated 55 vehicles from a convoy that gathered in areas southwest of Fallujah and a further 120 in an area northwest of the city.

“We know the Iraqi security forces destroyed more,” he said.

Iraq's Joint Operations Command said the air force destroyed 96 vehicles and killed “a large group” of fighters from June 29 to 30, while army aviation destroyed 507 and killed “dozens” over the same period.

While the JOC did not provide precise figures for the number of militants killed in Iraqi strikes, it said coalition bombing left 349 dead.

Massive IS convoys

It was not clear how the dead were counted and identified.

The defence ministry released aerial footage showing dozens of vehicles being targeted, and JOC spokesman Yahya Rasool said commandos had also seized large quantities of weapons and ammunition.

The strikes targeted massive convoys of IS vehicles including pickup trucks, minibuses and cars.

“This is a desperate attempt on the part of the terrorists to flee to their areas in Al-Qaim near the Syrian border and Tharthar,” said Anbar Operations Command chief Staff Major General Ismail al-Mahalawi.

Tharthar is a lake north of the Euphrates surrounded by desert through which IS fighters still have lines to reach Mosul, the country's second city and their last remaining major Iraqi hub.

Iraqi forces retook full control of Fallujah, just 50 kilometres west of Baghdad, after a vast operation that was launched in May.

After tough battles to breach IS defences in south Fallujah, elite Iraqi forces conquered the rest of the city with relative ease.

They took full control of the city on Sunday after IS fighters abandoned the Jolan neighbourhood and retreated to rural areas to the west.

The account of the air strikes provided by the JOC suggests IS fighters had no choice but to attempt a suicidal convoy that they knew would leave them exposed to air strikes.

According to Rasool and other military sources, the first strikes broke up a massive initial convoy that stretched several kilometres.

'A great victory'

Some left their vehicles and hid in a spot which was subsequently struck by Iraqi aircraft, resulting in a very high death toll, he said.

Fragments of the convoy were able to move on and some more vehicles were destroyed in subsequent strikes.

“We achieved a great victory by killing dozens of militants and the leaders of this organisation who tried to flee after their defeat,” Rasool said.

The JOC said the majority of the strikes were carried out by Iraqi aircraft and that US-led coalition warplanes joined the operation later.

It was not immediately clear whether some IS militants were able to survive the aerial onslaught and reach their strongholds near Syria.

The strikes appear to spell the end of fixed IS positions in eastern Anbar province, further shrinking the “caliphate” the group proclaimed over large parts of Iraq and Syria two years ago.

After losing the provincial capital Ramadi, as well as the towns of Heet and Rutba, defeat in Fallujah means the jihadist footprint in their traditional stronghold of Anbar is limited to areas near the Syrian border.

Iraqi forces are now training their sights on Mosul and pressing simultaneous operations from the south and the east of Qayyarah, a town in the Tigris valley they want to use as a launchpad for a full-fledged offensive on IS's de facto Iraqi capital.