AMERLI, Iraq (AFP) - Iraqi forces regained control on Tuesday of part of a key highway linking Baghdad to the north where Amnesty International said jihadists had carried out a wave of ethnic cleansing.

Iraqi troops backed by militia have been pushing north after breaking a months-long jihadist siege by Islamic State (ISIS) fighters of the Shi'ite Turkmen town of Amerli.

Visiting the town on Monday, outgoing prime minister Nuri al-Maliki vowed that the army would turn Iraq into a "graveyard" for ISIS.

A senior UN human rights official said the jihadist group had carried out "acts of inhumanity on an unimaginable scale," since it swept through much of the Sunni Arab heartland north of Baghdad in June and then stormed minority Christian and Yazidi Kurdish areas last month.

Amnesty International accused the jihadists of "war crimes, including mass summary killings and abductions".

"The massacres and abductions being carried out by the Islamic State provide harrowing new evidence that a wave of ethnic cleansing against minorities is sweeping across northern Iraq," said its senior crisis response adviser Donatella Rovera.

ISIS has carried out beheadings, crucifixions and public stonings in areas under its control in Iraq and neighbouring north-eastern Syria, where it has declared an Islamic "caliphate".

The breakthrough at Amerli on Sunday was the biggest success for the Iraqi government since the army's collapse across much of northern and north-central Iraq in June.

The United States carried out limited air strikes in the area during the Amerli operation, the first time it has expanded its more than three-week air campaign against ISIS outside the north.

On Tuesday, troops regained control of a stretch of the main highway to the north which had been closed by the miliants for almost three months, Army Staff Lieutenant General Abdulamir al-Zaidi said.

A senior militia commander said it would be several days before the road reopened as sappers needed to clear it of mines and booby-traps planted by the retreating militants.

On Monday, troops and Shi'ite militiamen retook Sulaiman Bek and Yankaja, two towns north of Amerli that had been important militant strongholds.

The government's reliance on Shi'ite militiamen in this and other operations risks entrenching groups which themselves have a history of brutal sectarian killings.

The US has said it had launched four air strikes in the Amerli area, meaning that it effectively supported operations involving militia forces that previously fought against US troops in Iraq.

General David Petraeus, a former commander-in-chief of US-led forces in Iraq, has warned against America becoming an "air force for Shi'ite militias".

Australian Prime Minister Tony Abbott on Tuesday said "extreme force" was justified in battling IS militants, comparing them to Nazis and communists.

Meanwhile, more pledges were made to provide arms to Iraq's Kurds, who are battling jihadists in the north and east.

Germany has announced that it will send anti-tank rocket launchers, rifles and hand grenades to support Kurdish forces.

German Chancellor Angela Merkel said Monday that an estimated 400 German nationals had travelled to Iraq and Syria to fight alongside jihadists, and that "we must fear these fighters could return one day".

Similar fears have been expressed by other Western governments. British Prime Minister David Cameron has announced tougher measures against suspected returning jihadists.

The United Nations mission to Iraq said on Monday that at least 1,420 people died in violence in August.

It said the figure did not include Anbar province, west of Baghdad, where the army has been battling militants all year.

ISIS and its allies control a large swathe of north-eastern Syria as well as territory in Iraq, and its rule has been marked by repeated atrocities, some of them videotaped and posted on the Internet.

"The reports we have received reveal acts of inhumanity on an unimaginable scale," deputy UN rights chief Flavia Pansieri said on Monday.

The UN Human Rights Council unanimously agreed to send an emergency mission to Iraq to investigate IS atrocities.

"We are facing a terrorist monster," Iraqi Human Rights Minister Mohammed Shia al-Sudani told the council, adding that IS had carried out acts "equivalent to genocide and crimes against humanity".

Washington has said operations in Syria will be needed to defeat ISIS, but has so far ruled out any cooperation with the Damascus regime.