DAMASCUS - Islamic State militants have executed more then 160 fleeing Syrian soldiers, a monitor said Thursday, the latest in a string of brutal abuses alarming Western powers who fear a global spread of the terror.

News of the killings comes as US President Barack Obama is reportedly weighing air strikes on IS positions in Syria and coming closer to greenlighting a mission to aid Shia Turkmen trapped in an Iraqi town by the militants.

Syrian regime air strikes killed six IS leaders on Thursday, the Syrian Observatory for Human Rights said, but Washington has so far baulked at cooperating with Damascus against the militants.

French President Francois Hollande added his voice to the disquiet that has been growing since the militants marauded through Iraq and beheaded US journalist James Foley.

The latest killings took place in the Syrian province of Raqa, the Syrian Observatory for Human Rights said, adding that the victims were soldiers fleeing towards government-held territory to the west after the militants overran their base at Tabqa.

The militants boasted on Twitter that they had killed 200 defeated troops and posted video of what they said was the garrison in headlong flight.

"IS executed more than 160 Syrian soldiers in three different places in Raqa province yesterday and at dawn today," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

The militants seized the airport on Sunday after weeks of bitter fighting with loyalist forces, cementing their control over Raqa province, capital of their self-declared Islamic "caliphate".

Abdel Rahman said the defeated garrison comprised 1,400 soldiers, 200 of whom were killed and 700 of whom managed to escape.

Of the other 500, dozens were captured on Wednesday night as they attempted to cross the desert to government-held territory in the Orontes Valley to the west.

Meanwhile, the United Nations confirmed that an armed group captured 43 UN peacekeepers on the Syrian side of the Golan Heights on Thursday, saying it was doing everything to secure their release. "Forty-three peacekeepers from the United Nations Disengagement Observer Force (UNDOF) were detained early this morning by an armed group in the vicinity of Quneitra," it said in a statement.

An additional 81 peacekeepers were "currently being restricted to their positions in the vicinity of Ar Ruwayhinah and Burayqah," it added.

No details were released on the nationalities of the detained peacekeepers but six countries are contributing to the 1,200-strong unit: Fiji, India, Ireland, Nepal, Netherlands and the Philippines.

IS posted video footage showing young men in underwear being marched barefoot along a desert road. Militants shouted "Islamic State" and "There's no going back".

A UN-mandated probe charged Wednesday that public executions, amputations, lashings and mock crucifixions have become a regular fixture in militant-controlled areas of Syria.

The Syrian air force hit back on Thursday with a strike on a house in the eastern town of Mu-Hassan where IS leaders were meeting, killing six of them, the Observatory said.

The UN has also highlighted the plight of the thousands of mainly Shia Turkmen residents of the northern Iraq town of Amerli, who face danger both because of their faith, which militants consider heresy, and their resistance against the militants, of the sort that has drawn deadly retribution elsewhere.

The town has been besieged for more than two months and residents are desperate for food and water and fear a massacre if the militants push through their defences.

Washington is weighing both aid drops and air strikes to help the town, US officials said on Wednesday.

"It could be a humanitarian operation. It could be a military operation. It could be both," a US defence official said, speaking on condition of anonymity.

Iraq is preparing its own effort, massing forces north and south of the town in Salaheddin province and carrying out air strikes against the militant militants besieging it.

There is "no possibility of evacuating them so far", Eliana Nabaa, spokeswoman for the UN mission in Iraq, said of Amerli residents.

UN Iraq envoy Nickolay Mladenov has called for an urgent effort to help Amerli, saying residents face a "possible massacre" if the town is overrun.

The United States, which has been carrying out an air campaign against IS militants in Iraq since August 8, has begun surveillance flights over Syria too, a possible precursor to air strikes.

The US focus on Syria comes after President Bashar al-Assad's regime said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters.

French President Hollande launched a bitter tirade against Assad on Thursday for his suggestion the West work with his regime to defeat the militants.

"Assad cannot be a partner in the fight against terrorism, he is the de facto ally of militants," Hollande said.

He renewed his proposal to host an international conference "to organise the coordination of international action against the Islamic State on humanitarian, security and military fronts".

The growing numbers of Westerners joining IS and other extremist groups has raised fears that they could return home and carry out attacks.

US officials have confirmed that American Douglas McCain, 33, was killed in Syria fighting for the Islamic State, and were investigating reports that a second American had been killed fighting for Al-Qaeda's Syria affiliate.

France's president rejected Thursday any cooperation with Bashar al-Assad whom he accused of being a "de-facto ally" of militants wreaking havoc in Syria and Iraq, and urged a "humanitarian and military" response to the threat.

Speaking at an annual Paris gathering of ambassadors from around the world, Francois Hollande touched on crises in both countries as well as those rocking Ukraine and Libya - all of which he said affected France directly.

"Assad cannot be a partner in the fight against terrorism, he is the de-facto ally of militants," he said.

Hollande reiterated his proposal to hold an international conference in Paris "to organise the coordination of international action against the Islamic State on humanitarian, security and military fronts".

His comments came after Assad's regime said Monday it was willing to work with the international community, including Washington, to tackle IS militants in the war-ravaged country.

The United States has already carried out reconnaissance flights on Islamic State positions in Syria in surveillance seen as a precursor to possible strikes against the radical group, whose campaign of extreme violence has terrorised civilians and rivals alike.

Meanwhile, Lebanese troops clashed with gunmen Thursday near the Syrian border where there was deadly fighting with rebels from over the frontier three weeks ago.

The fighting, which erupted before dawn, was still raging in late afternoon, with troops shelling the gunmen who had taken up position on the outskirts of the town, an AFP correspondent reported.

There was no immediate word on casualties but the correspondent saw a wounded soldier being evacuated in a military vehicle as plumes of black smoke billowed into the sky over the town.