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Iraqi forces backed by US-led coalition air strikes have opened supply lines into the town of Beiji and its nearby oil refinery.

The development came as the UN condemned alleged abuses of the Islamic State of Iraq and the Levant (ISIL) group against women.

Colonel Steve Warren, the defence department spokesman, said the US was not ready to declare Beiji refinery, the country's biggest, or the town "liberated", but progress was being made by Iraqi troops.

Divisions rife as Iraq struggles against ISIL

"Friendly forces have begun moving into the town ... and are methodically beginning to root out the enemy," Warren said.

At the refinery, Iraqi troops broke through to a contingent of Iraqi troops dug in at the northwest corner of the facility for several months and were able to deliver equipment and personnel to the men on the ground.

"This was a methodical and often times grinding movement from the south to the north," Warren said.

"It took the friendly forces several weeks to be able to break through" an ISIL defensive line, the Pentagon spokesperson was quoted by Reuters news agency as saying.

The Iraqi force is comprised of a mix of conventional troops and the Popular Mobilisation Forces, which comprise Shia units who answered a call by political and religious leaders to take on ISIL after its advances last year.

The US refuses to back the Shia units, which are supported by Iran, and have faced accusations of atrocities against Sunni civilians.

Sexual abuses

The battlefield advances came as the UN envoy on sexual violence, Zainab Bangura, criticised ISIL for its alleged abuses against women.

Teenage girls abducted by ISIL fighters in Iraq and Syria are being sold in slave markets, Bangura said in an interview with the AFP news agency on Monday.

"This is a war that is being fought on the bodies of women." 

Yazidi woman talks about ISIL captivity

Bangura spoke to women and girls who had escaped from captivity in ISIL-controlled areas during a visit to Iraq and Syria in April.

Reports indicate the fighters continue to run slave markets for girls abducted during fresh offensives, but there are no figures on the numbers enslaved by the fighters.

"They kidnap and abduct women when they take areas so they have, I don't want to call it a fresh supply, but they have new girls," she said.

Girls are sold for "as little as a pack of cigarettes" or for several hundred or thousand dollars, she said.

Bangura described the ordeal of several teenage girls, many of whom were part of the Yazidi minority targeted by ISIL.

"Some were taken, locked up in a room, over 100 of them in a small house, stripped naked and washed," Bangura said.

Abducting girls has become a key part of the ISIL strategy to recruit foreign fighters who have been travelling to Iraq and Syria in record numbers over the past 18 months.