The charity that employs British aid worker David Haines, threatened with death by Islamic State militants in a recent video, called on Friday for his immediate release.
The Agency for Technical Cooperation and Development (ACTED) said it was "deeply shocked by the images broadcast earlier this week. The threats on David Haines' life are intolerable."
Haines was threatened with death by the Islamic State militants during a video depicting the beheading of US journalist Steven Sotloff.
ACTED said the 44-year-old Haines, originally from Scotland, had been a humanitarian worker since 1999, helping victims of conflict in the Balkans, Africa and the Middle East.
"When he was taken hostage in March 2013, David was in Syria as part of ACTED's humanitarian effort in support of tens of thousands of people affected by the crisis," the charity said.
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ACTED said it "strongly condemns the violence and threats" against Haines, adding that "a man's life should never be threatened on account of his humanitarian commitment.
"We call for the immediate release of David," it said.
IS recently posted video footage on the Internet of Sotloff's beheading, confirmed as authentic by Washington, which sparked outrage around the world.
It said the journalist's killing, which came on the heels of its beheading last month of another US reporter, James Foley, was in retaliation for expanded US air strikes against its fighters in Iraq.
The masked executioner in the video spoke with a London accent and claimed to be the same man, confirmed by UK security services as a Briton, who beheaded Foley.
At the end of the five-minute recording, the militant threatens another captive, identified as Haines.