BEIRUT - Islamic State fighters have executed dozens of members of the Syrian army they took hostage after capturing an air base in the northeast of the country, a group monitoring the violence said on Thursday.

Islamic State, a radical offshoot of al-Qaida, stormed Tabqa air base on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the fiercest confrontations yet between the two sides.

The Syrian Observatory for Human Rights, which monitors the conflict using sources on the ground, said the soldiers who were executed had been trying to escape from the airport when they were taken hostage by militant fighters of the Islamic State.

A video posted online by Islamic State supporters on Thursday appeared to show members of the group making scores of Syrian army hostages walk and run through the desert in their underwear.

Reuters could not confirm the contents of the video, which was posted on You Tube and social media. It showed at least 135 men, some with their hands on their heads, running barefoot through a desert landscape as armed men jeered them.

It was not clear what happened to the men afterwards, but photos posted by Islamic State supporters online on Wednesday appeared to show them gunning down at least seven members of the army they had taken hostage.

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