The Iraqi government will investigate allegations of abuses by the security forces in the course of the operation to retake the jihadist-held city of Fallujah, a spokesman said on Sunday.

Prime Minister Haider al-Abadi has ordered the creation of a human rights committee to examine "any violation to the instructions on the protection of civilians", Saad al-Hadithi said in a televised briefing.

He said Abadi had issued "strict orders" for prosecutions to take place in the event of any abuses.

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

Officials including Parliament Speaker Salim al-Juburi 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 on the alleged abuses, but urged Abadi to "look into these acts and deal with them in a strict and expeditious way".

Fallujah is a Sunni city that lies only 50 kilometres (30 miles) west of Baghdad and is one of IS's most emblematic bastions.

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

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

Those groups have been repeatedly accused of fuelling sectarianism and their involvement in the Fallujah battle was seen as potentially explosive.

Hadithi reiterated Abadi's stance that the fate of the estimated 50,000 civilians still believed trapped inside the besieged city was given utmost priority.

"The operation to liberate Fallujah could have been completed in days but we put the safety of civilians first," the spokesman said.

Iraqi forces launched an offensive to retake the jihadist bastion on May 22-23. After a week of shaping operations, they have since struggled to break into the city centre.

The UN's top envoy in Iraq, Jan Kubis, also said in a statement that Iraq should "thoroughly investigate" reports of human rights violations against civilians.

"This noble cause of ridding Fallujah of Daesh (IS) terrorists should not be allowed to be tarnished by violations of human rights and dignity of people, notably on sectarian grounds," he said.

Iraqi forces find mass grave near Fallujah: officials
Baghdad (AFP) June 5, 2016 - Iraqi forces involved in operations to retake the city of Fallujah on Sunday found a mass grave believed to contain 400 bodies, a provincial official and a senior officer said.

A colonel in the police of Anbar, where Fallujah is located, said the mass burial site was discovered in the town of Saqlawiya, northwest of Fallujah in the Euphrates Valley.

"The security forces of the federal police, the army and the Hashed al-Shaabi found a mass grave in the Shuhada neighbourhood during a mine clearing operation," he said.

"The mass grave contains about 400 bodies of members of the military. There are also some civilians," he said, speaking on condition of anonymity because he is not authorised to talk to the press.

The colonel said most of the victims appeared to have been shot in the head.

"The security forces have opened the mass grave and started transferring the bodies for identification," he said.

The bodies are believed to belong mostly to Iraqi soldiers who were massacred by the Islamic State group in a series of devastating attacks on army bases in the area.

"The organisation (IS) executed many members of the military, as well as civilians, in this area in late 2014 and early 2015," he said.

Rajeh Barakat, a member of the Anbar provincial council, confirmed the discovery.

"The mass grave also includes civilians executed by Daesh (IS) on various charges such as spying or breaking the organisation's rules, " he said.

Iraqi forces launched an operation to retake Fallujah -- one of the jihadists' most emblematic bastions -- two weeks ago.

The offensive included operations to seal the siege of Fallujah by retaking the smaller town of Saqlawiya and its surroundings, a phase which has nearly been completed.