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The mass burial site was discovered in the town of Saqlawiya, northwest of Fallujah, after its recapture from Islamic State militants by the Iraqi army and the Popular Mobilisation Forces [PMF] on Saturday.

"The security forces of the federal police, the army and the [PMF] found a mass grave in the Shuhada neighbourhood during a mine-clearing operation," reported a colonel in the Anbar provincial police force.

"The security forces have opened the mass grave and started transferring the bodies for identification. It contains about 400 bodies of members of the military. There are also some civilians," he said.

Most of the victims appeared to have been shot in the head, he added.

Most of the corpses uncovered are believed to be those of Iraqi soldiers massacred by IS militants in a series of attacks on army bases in the area.

"[IS] executed many members of the military, as well as civilians, in this area in late 2014 and early 2015," the colonel said.

A member of the Anbar provincial council confirmed the discovery.

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"The mass grave also includes civilians executed by Daesh on various charges, such as spying or breaking the organisation's rules," said Rajeh Barakat, using the Arabic acronym for IS.

The Iraqi forces, backed by Shia militias from the PMF, last month launched an operation to retake Fallujah city - just 50km from Baghdad - which became an IS group stronghold after militants took control there in January 2014.

But earlier this week the UN warned that as many as 50,000 civilians were trapped in the city and had been prevented from fleeing the violence by IS militants.

These claims were echoed on Sunday by the  who said that IS militants were shooting and killing civilians that tried to flee Fallujah.

"Reports from families that NRC has been in touch with describe that civilians trying to cross the Euphrates River in order to flee the fighting are being targeted by armed opposition groups," the organisation said in a statement.

"An unidentified number of civilians have been shot and killed trying to cross the river," the NRC said.

Eighteen bodies, including seven belonging to children, were pulled out of the river on Saturday, sources told °®Âþµº.

Our biggest fears are now tragically confirmed, with civilians being directly targeted while trying to flee to safety -Norwegian Refugee Council

"Our biggest fears are now tragically confirmed, with civilians being directly targeted while trying to flee to safety," said NRC country director Nasr Muflahi.

"This is the worst that we feared would happen to innocent men, women and children who have had to leave everything behind in order to save their lives," he added.

Around 18,000 civilians have reached displacement camps since Iraqi forces began their offensive to retake Fallujah two weeks ago.

Abuse allegations 

The Iraqi government on Sunday announced it would investigate allegations of abuse by its security forces in the course of the operation to retake the IS-held city.

Prime Minister Haider al-Abadi ordered the creation of a human rights committee to probe "any violation to the instructions on the protection of civilians", spokesman Saad al-Hadithi said.

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Abadi had issued "strict orders" for prosecutions to take place in the event of abuses, Hadithi added.

Grand Ayatollah Ali al-Sistani, one of the most revered Shia clerics in Iraq, has also issued guidelines intended as a code of conduct for forces fighting the Islamic State group, aimed at curbing abuses.

Officials, including parliament speaker Salim al-Jubouri, have expressed concern over reports of abuses committed by the forces involved in the operation to recapture Fallujah.

Juburi spoke on Thursday of "information indicating that some violations were carried out by some members of the federal police and some volunteers against civilians".

The statement did not provide details, but urged Abadi to "look into these acts and deal with them in a strict and expeditious way".

Fallujah is a majority Sunni city just 50km [30 miles] west of Baghdad, and has become one of IS' most emblematic strongholds.

The PMF taking part in the Fallujah operation is an umbrella organisation that includes Sunni tribal fighters but is dominated by powerful Tehran-backed Shia militias.

It is nominally under Abadi's authority - but some of its most powerful groups are thought to answer directly to Iran.

These militias have been repeatedly accused of fuelling sectarianism when involved in battles against IS across the country, and their involvement in the Fallujah battle was seen as potentially explosive.

Agencies contributed to this report.