Anti-ISIS coalition says it killed militant linked to hostage beheadings

Analysts warn that fully eradicating ISIS from Syrian desert hideouts could prove almost impossible

The US-led coalition says it has targeted a senior member of ISIS involved in the 2014 killing of American aid worker Peter Kassig. AP

The US-led coalition against ISIS said Monday it killed a senior militant involved in the executions of an American aid worker and other Western hostages.

Abu Al Umarayn was accused of involvement in the November 2014 beheading of Peter Kassig, a former US ranger who was doing volunteer humanitarian work when captured in 2013.

"He was killed and more information will be available after a full assessment," Sean Ryan, spokesman for the US-led coalition, said in a statement issued after the Sunday strikes.

"Al Umarayn had given indications of posing an imminent threat to coalition forces and he was involved in the killing of American citizen and former US Army Ranger, Peter Kassig," he said.

Ryan said the militant had also been involved in the execution of several other prisoners.

It is the first time the coalition, which has been hunting down ISIS fighters in Iraq and Syria since 2014, has announced the killing of a militant leader linked to Kassig's death.

At the time of the execution, ISIS released a video showing Kassig's severed head but did not publish footage of the decapitation, as it had done for other hostages.

Syria's official Sana news agency had earlier Sunday accused the US-led coalition of firing on Syrian army positions in remote eastern regions.

"The American coalition forces launched around 8:00 pm this evening several missiles against some positions of our forces in the Ghorab mountains south of Sukhna," it said.

Quoting a military source, it said the bombardment had caused only material damage.

The Syrian Observatory for Human Rights said coalition forces fired "more than 14 missiles" at a Syrian army convoy as it was passing through the desert.

"The group was lost in the middle of the desert around 35 kilometres from the Al Tanf base," the Observatory's director Rami Abdel Rahman said.

The United States often uses this base to launch its strikes against ISIS.

Coalition spokesman Sean Ryan denied any strikes targeted the Syrian army.

"False, the strikes were as the report stated and directed at ISIS," he said.

Kassig founded a humanitarian organisation in 2012 that trained some 150 civilians to provide medical aid to people in Syria. His group also gave food, cooking supplies, clothing and medicine to the needy.

He took the name Abdul Rahman after converting to Islam.

His execution was part of a gruesome series of Western hostage beheadings that ISIS filmed and published to shock the world as it attempted to expand across the region.

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Before Kassig's decapitation, which ISIS announced on November 16, four other hostages were executed by ISIS: British aid workers Alan Henning and David Haines; and US journalists Steven Sotloff and James Foley.

Another hostage held at the time and whose execution was threatened was British journalist John Cantlie.

He later appeared in videos in which he repeated ISIS propaganda but, more than six years after his kidnapping, his fate remains unclear.

The leader of the cell which was responsible for the executions and became known as "The Beatles" was believed to be Mohammed Emwazi, a British jihadist nicknamed "Jihadi John" who was killed in a drone strike in 2015.

After expanding to control a self-styled "caliphate" straddling Syria and Iraq which was larger than Britain, ISIS suffered a string of military setbacks.

It has virtually no fixed positions left in Iraq and is now defending a few pockets in desert areas of Syria, including the region where Sunday's strikes were carried out.

The other is the militants' main active front in the Hajin area of Deir Ezzor province, where coalition-backed Kurdish-led fighters have been struggling to flush out a group of die-hard militants making a fierce last stand.

The coalition as well as the Syrian government and its Russian backers have all repeatedly vowed to carry on the fight until achieving a full victory over ISIS.

But analysts have warned that fully eradicating the militants from those desert hideouts where the state has a very limited footprint could prove almost impossible.