The Islamic State (IS) executed the first hostage of the people who were abducted from Syria's southern province of Sweida during a recent attack, a monitor group reported on Sunday.

Muhannad Abu Ammar, 19 years old, was among the 30 people abducted by IS during their sudden attack on towns in the eastern countryside of Sweida province on July 25.

In a video footage posted online by IS, Ammar was shown ahead of his execution.

The execution took place on Thursday, according to the video.

Meanwhile, the Syrian Observatory for Human Rights war monitor said the reason for his slaughter was the failure of the negotiations between IS and the Syrian forces.

The negotiations sought the release of the hostages in exchange for the government forces' acceptance to allow tens of IS militants to leave the western countryside of Daraa province toward the Syrian desert.

The Britain-based watchdog group said fear has risen for the lives of the other hostages taken by IS during the attack in Sweida, which killed over 255 people.