Iraqi Kurdish forces on Wednesday launched a broad offensive backed by mass bombing from US-led coalition warplanes to retake the northeastern Sinjar area from the Islamic State group.

In neighbouring Syria, the bodies of 230 people from a tribe that rose up against the jihadists in the Deir Ezzor region have been found in a mass grave, a monitoring group said.

Sixty-one air strikes were carried out in Iraq since Monday, some of the heaviest bombardment since a jihadist onslaught on the Sinjar region prompted the first US air raids four months ago.

The US military said "45 strikes were conducted in support of the peshmerga (Kurdish army) and Iraqi security forces operating in the region".

Most of the other raids were also in northern Iraq, destroying dozens of bulldozers, vehicles, checkpoints, enemy fighting positions, enemy fighters, and equipment.

The peshmerga said eight areas had been recaptured in the initial phase of its offensive on Wednesday and around 80 IS fighters killed.

On social media, Kurdish fighters posted pictures of each other posing in front of dead bodies.

Among the goals of the operation is the Sinjar area, home to many members of the Yazidi minority before jihadists attacked in early August and forced most of them to flee.

The risk of a genocide against the Yazidis was one of the reasons US President Barack Obama put forward for launching the aerial campaign against IS.

- Syria mass grave -

Wednesday's operation was launched from Rabia, on the border with Syria, and Zumar, on the shores of Mosul dam lake, said senior Kurdish officers.

"Peshmerga forces launched an operation to liberate some important areas in Sinjar and Zumar at 7:00 am (0400 GMT)," a peshmerga brigadier general said.

Masrour Barzani, chancellor of the Kurdistan Regional Security Council, said the offensive showed the extremist organisation that proclaimed a "caliphate" over parts of Iraq and Syria six months ago could be defeated.

"While IS projects strength through a campaign of intimidation and brutality the peshmerga have exposed its weakness today, demonstrating once again that it can be defeated," he said.

The jihadists have suffered a string of recent setbacks in Iraq, but the frontlines have been more static in Syria.

The Syrian Observatory for Human Rights said Wednesday it had "learned from trusted sources that more than 230 bodies have been found in a mass grave in the desert near Al-Kashkiyeh."

The dead were from the Shaitat tribe, which rose up against IS in the summer.

The Observatory said 900 members of the tribe were now confirmed to have been slain, while hundreds more were missing.

The bodies were discovered by relatives as they returned to their villages from months of displacement after agreeing to respect an IS-imposed curfew.

The 60-nation US-led coalition against IS, which also carried out five air strikes in Syria this week, is not directly cooperating with the Syrian regime.

Instead it seeks to support moderate rebels, who are opposed both to President Bashar al-Assad and the jihadists but have limited man and fire power.

The administrations in Baghdad and in the autonomous Iraqi region of Kurdistan however have received strong support from Western nations, in the form of weapons, aid and thousands of military advisers.

- Humanitarian concerns -

Kurdish leaders said one of the objectives of Wednesday's offensive was to "break the siege on Mount Sinjar".

The 60-kilometre-long (40 miles) range was the scene in August of one of the most dramatic episodes of the assault on Iraq the jihadists launched in early June.

IS killed hundreds of residents, abducted and enslaved hundreds of Yazidi women and girls and forced tens of thousands to seek refuge on Mount Sinjar.

Civilians remained besieged there for days in the searing summer heat with little to eat or drink before a breach in the siege allowed them to flee.

Yazidi fighting units that were formed in the wake of the August attack have struggled to control land in the Sinjar area and retreated to the mountain in September.

A few thousand people are still atop Mount Sinjar. Some are residents of the mountain but most are fighters, from various regional Kurdish groups or recently-formed Yazidi militias.

"We are ready, we are waiting for the advancing forces from Rabia and Zumar," said Dawood Jundi, a peshmerga field commander.

With the winter setting in, concerns have grown over the fate of the roughly 10 million people who are internally displaced in Iraq and Syria.

The UN Security Council on Wednesday agreed to extend for a year cross-border deliveries of desperately-needed humanitarian aid to rebel-held areas in Syria.

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