Daesh killed a 19-year-old male student who was among more than 30 people, most of them women and children, from a village in the southern province of Sweida who were taken hostage last week.
The Daesh militant group has executed one of dozens of Druze hostages abducted from Syria's southern province of Sweida last week, a journalist in the area and a monitor said on Sunday.
The killing prompted an offer from the US-backed Syrian Democratic Forces (SDF) to exchange captured militants for the remaining Druze civilians.
Daesh went on a rampage in Sweida on July 25, killing more than 250 people – mostly civilians – in the deadliest attack ever to target the mostly government-held province and its Druze religious minority.
The militants also kidnapped more than 30 people, most of them women and children, from a village in the province that had previously remained largely isolated from Syria's seven-year civil war.
On Thursday, Daesh killed a 19-year-old male student who was among the hostages, the head of the Sweida24 news website Nour Radwan told AFP.
Quoting relatives, Radwan, who was speaking from Sweida, said the young man was taken from the village of Al Shabki on July 25 along with his mother.
His family received two videos, the first showing him being decapitated and the second of him speaking before being killed as well as images of his body after his death, Radwan said.
Sweida24 posted online part of a second video, seen by AFP, showing a bearded young man who appeared to be sitting on the ground in a landscape of grey rocks.
He is wearing a black T-shirt and tracksuit bottoms, and his hands are tied behind his back.
The video could not be independently verified.
Daesh has not claimed the kidnappings and did not publish the video on its usual channels.
'Failure of talks'
Daesh has lost much of the territory they once controlled in Syria after overrunning large swathes of it in 2014, but they retain a presence in the east of the country and in the country's vast Badiya desert.
The regime has been fighting in recent weeks to expel Daesh militants from a patch of the neighbouring province of Daraa.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said the young man's execution was the first since the kidnappings.
The execution came "after the failure of talks between Daesh and regime forces over the transfer of Daesh militants from the southwest of Daraa province to the Badiya" desert, the Observatory said.
It also follows the execution of 50 Daesh militants and civilians in Daraa province earlier in the week at the hands of rebels, according to the monitor, which relies on a network of sources inside Syria.
Stepping in with an offer on Sunday, the SDF, whose main bulk of militants is comprised of PKK-linked YPG members, told AFP that it was "fully prepared" to exchange Daesh militants for the Sweida civilians.
"We assure our people in the town of Sweida and the families of the hostages that we are fully prepared for any exchange with Daesh," SDF spokesman Redur Khalil told AFP.
"This initiative does not replace Russia's mediation," he said.
On Friday, a top Druze religious leader said Syrian regime ally Russia was in talks with Daesh over the release of those abducted in Sweida.
Daesh see members of the secretive Druze sect, who made up three percent of Syria's population before its war broke out in 2011, as heretics.
Syria expert Khattar Abu Diab said that the kidnappings in Sweida marked a turning point for the minority.
"For this ancestral community, the abduction of women oversteps all red lines," he said.
"Their reaction will depend on the outcome of negotiations but if all the hostages were killed," the Druze could directly intervene to expel Daesh from the desert, he said.
Regime forces have in recent weeks ousted Daesh from all of the towns and villages in the Yarmouk Basin in the northwest of Daraa province.
Syrian regime media have said regime troops are pursuing the last remaining militants who fled to nearby valleys.
In areas retaken by the Russia-backed regime, the regime has sometimes negotiated to take back control of land in exchange for the transfer of fighters to other parts of Syria.
During the July 25 attack in Sweida, Daesh abducted 36 Druze women and children from a village in Sweida's east, the Observatory said at the time.
Four women have since escaped while two have died, leaving 14 women and 16 children in Daesh captivity, according to the Observatory.
At the time, another 17 men were unaccounted for, but it was unclear if they were also kidnapped.
Local sources say the abductees' families have been sent photos and videos of their loved ones via WhatsApp.
The Sweida killing is the first such execution of a kidnapped civilian by Daesh since they overran the town of Al Qaryatain in central Syria for several weeks in October last year, the Observatory said.