Kurdish forces backed by foreign jets pressed an ambitious operation against the Islamic State group in northern Iraq on Thursday, taking on the jihadists in the heart of their "caliphate".

After mass bombing by aircraft of the US-led coalition paved the way for ground troops Wednesday, Iraq's Kurdish peshmerga retook several villages and closed in on the Sinjar area.

The anti-IS war's top commander, US Lieutenant General James Terry, said Thursday that 50 air strikes had allowed the Kurds to "regain approximately 100 square kilometres of ground".

IS captured Sinjar in early August, and preventing a genocide against its largely Yazidi minority was a reason US President Barack Obama put forward for launching the air war against the jihadists.

Fresh strikes were conducted on Thursday north of Tall Afar, one of the first areas to fall to IS fighters in early June, said Anwar Brahim, a senior officer with the Kurds' intelligence services.

"At around 7:00 am (0400 GMT), there were coalition strikes on Nahyat al-Ayadhiya," Brahim told AFP. "A large deployment of peshmerga is ready to close in on Sinjar."

An AFP reporter saw the mangled bodies of IS fighters killed by the air strikes and which peshmerga fighters covered with sand to reduce the stench.

Kurdish forces used heavy artillery to pound IS positions in the area but officials said the fighting was less intense than on Wednesday.

In Hanakeh, one of the villages the peshmerga took back, one bridge had been blown up and another still had four barrels filled with TNT on it, suggesting the jihadists had to flee before they had time to destroy it.

The US military command supervising the coalition air campaign said Wednesday that 61 air strikes had been carried out in Iraq since Monday.

That was some of the heaviest bombardment since the jihadist onslaught on the Sinjar area and towards Iraq's autonomous Kurdish region prompted the first US air raids four months ago.

- Dozens killed -

The peshmerga said they recaptured eight villages and killed about 80 IS fighters in the initial phase of the offensive launched from Rabia on the Syria border and Zumar on the shores of Mosul dam lake.

The peshmerga lost seven men on Wednesday in Qasreej village when they failed to stop a suicide attacker who rammed an explosives-laden armoured vehicle into their convoy, officers told AFP on the scene.

Sinjar, between the Syria border and Iraq's IS-held second city Mosul, is one of the main goals of the offensive.

The operation aims to disrupt IS supply lines and further isolate Mosul, around which the jihadists have been building berms and trenches in recent weeks.

IS has also cut the mobile phone network in the city and imposed tougher restrictions on the population's movements, sparking accusations they are seeking to use civilians as human shields in the event of an attack.

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

The Syrian Observatory for Human Rights said Wednesday it had learnt that a mass grave was uncovered with the bodies of 230 people from the Shaitat tribe which rose up against IS in the summer in the Deir Ezzor area.

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

- Second siege -

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

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

Kurdish leaders said one of the objectives of Wednesday's offensive was to "break the siege on Mount Sinjar", which was the scene in August of one of the conflict's most dramatic episodes.

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 in what Yazidi leaders have called a "second siege".

Some are residents of the mountain but most are fighters, from various regional Kurdish groups or recently-formed Yazidi militias.

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