Damascus (AFP) - Islamic State jihadists have executed "dozens" of 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 U.S. President Barack Obama is reportedly weighing air strikes on IS positions in Syria and coming closer to greenlighting a mission to aid Shiite Turkmen trapped in an Iraqi town by the jihadists.

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

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

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

"Dozens of Syrian soldiers captured while fleeing... after the IS overran Tabqa airbase were executed by the jihadists during the night," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

The jihadists 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.

The other 500 remain on the run. Dozens were captured on Wednesday night as they attempted to cross the desert to government-held territory in the Orontes Valley to the west.

A militant Islamist fighter uses a mobile to film his fellow fighters taking part in a military parade along the streets of Syria's northern Raqqa province June 30, 2014. REUTERS/Stringer A Forced March Through The Desert A Forced March Through The Desert

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 jihadist-controlled areas of Syria.

The UN has also highlighted the plight of the thousands of mainly Shiite Turkmen residents of the northern Iraq town of Amerli, who face danger both because of their faith, which jihadists 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 jihadists push through their defences.

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

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

Iraq is also preparing its own effort, massing forces north and south of the town in Salaheddin province and carrying out air strikes against the jihadist 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 U.S. 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.

Syria's President Bashar al-Assad speaks during an interview with Venezuelan state television TeleSUR in Damascus, in this handout photograph distributed by Syria's national news agency SANA on September 26, 2013. REUTERS/SANA France Continues To Blast Assad France Continues To Blast Assad

But U.S. officials said they did not plan to coordinate with Damascus on targeting IS militants in Syria, despite Syrian insistence that any military action on its soil must be discussed in advance.

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

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

U.S. 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.