A broad peshmerga operation backed by US-led air strikes broke the Islamic State group's weeks-old siege of Iraq's Mount Sinjar on Thursday, a top Iraqi Kurdistan security official said.

"Peshmerga forces have reached Mount Sinjar, the siege on the mountain has been lifted," Masrour Barzani, the chancellor of the Kurdistan Regional Security Council, told reporters from an operations centre near the border with Syria.

A Yazidi leader atop the mountain however said he could see no sign of a military deployment and a peshmerga commander explained that any evacuation would only begin on Friday.

A statement from Barzani's office said the operation launched by the peshmerga on Wednesday morning had been one of the most successful so far against the Islamic State (IS) group.

It said the siege was broken at 1530 GMT and was the conclusion of an operation involving 8,000 peshmerga forces.

"This operation represents the single biggest military offensive against IS and the most successful," the statement said.

It also said that, as a result, jihadist fighters had fled en masse towards strongholds such as Tall Afar and Mosul, Iraq's second city and the main IS hub in northern Iraq.

Barzani told reporters the peshmerga push had cut key supply lines used by IS and reclaimed a total of 700 square kilometres (270 square miles) during the past two days.

A devastating jihadist attack on the Yazidi minority's Sinjar heartland in August displaced tens of thousands of people and was one of the reasons put forward by US President Barack Obama for launching strikes four months ago.

- 'Corridor secured' -

Amid fears of a genocide against the small Kurdish-speaking minority, tens of thousands of Yazidis fled to the mountain and remained trapped there in the searing summer heat with no supplies.

Kurdish fighters, mostly Syrian, broke that first siege but remaining anti-IS forces were subsequently unable to hold positions in the plains and retreated back to the mountain in late September.

The peshmerga commander for the area said troops had reached the mountain and secured a road that would enable people to leave, effectively breaking the siege.

"Tomorrow most of the people will come down from the mountain," Mohamed Kojar told AFP by phone, explaining the offensive had secured a corridor northeast of the mountain.

Dawood Jundi, a peshmerga field commander based on the mountain, also said the road had been secured but said some pockets of IS presence near the peshmerga advance remained to be cleared.

Said Hassan Said, a Yazidi politician also based on the mountain, for his part said he could see no sign that the siege had been broken.

"I'm on top of the mountain right now, I can see all areas from my position," he said. "There are no clashes, no movements, there is no deployment of peshmerga I can see."

Said put the number of families on the mountain at 1,200 and stressed they were surviving on fast dwindling supplies.

Speaking to reporters near the Fishkhabur border crossing with Syria, about 80 kilometres (50 miles) northwest of the mountain, Barzani said the operation had a key road used by IS fighters east of their main Iraqi hub of Mosul.