The charity which employs former Perthshire man David Haines has said it is “deeply shocked” by the video of him broadcast this week in which he is threatened with beheading and called for his immediate release.
ACTED said that the threats made to the ex-Scone man’s life were “intolerable”.
Mr Haines, 44, was taken while working for ACTED in Syria in March 2013, having previously helped local people in Libya and South Sudan.
In a statement the charity said: “ACTED strongly condemns the violence and threats against David. A man’s life should never be threatened on account of his humanitarian commitment.”
In its statement, ACTED said Mr Haines had worked as “a humanitarian” since 1999, helping people in the Balkans, the Middle East and Africa. When he was taken by jihadists in Syria he was working to help “tens of thousands of people affected by the crisis” created by the long-running civil war.
It added: “ACTED is deeply shocked by the images broadcast earlier this week. The threats on David Haines’ life are intolerable.
“We have been mobilised from day one with David’s family. More than ever, we are pursuing our efforts, and our thoughts are with David and his family.”
Militants from Islamic State also referred to as Isil or Isis have threatened to kill Mr Haines, who they are believed to be holding in Syria.
They have already beheaded two American journalists, posting the evidence online in gruesome videos featuring a masked jihadist with a British accent.
Mr Haines has a teenage daughter in Scotland from a previous marriage and a four-year-old daughter in Croatia from his present marriage.
Educated at Perth Academy, he has worked for aid agencies in some of the world’s worst trouble spots.
He was in Libya during its civil war in 2011, working as head of mission for Handicap International, which helps disabled people in poverty and conflict zones around the world.
In 2012 Mr Haines worked in South Sudan for the Brussels charity Nonviolent Peaceforce, which sends unarmed civilian peacekeepers to conflict zones.
In a statement it said he acted as a “non-partisan unarmed civilian peace facilitator” in the fledgling African state.
“We join with people around the world in pleas, thoughts and calls for his safe release,” it said.
A neighbour in Scone said Mr Haines had been based in the village until he separated from his wife several years ago.
She added: “It’s terrible. We are all in shock. He hadn’t lived here for years but it’s just so unexpected they are saying he is next.”
Perth councillor Douglas Pover, who helped run the army cadets at Perth Academy for many years, said: “It really brings it home when it’s a local lad that I may have known in the past involved in a high-profile international incident.
“Obviously, my thoughts go out to the family.”
Prime Minister David Cameron has warned the West must act “carefully and methodically” to deal with the threat of Islamic State amid a growing clamour for Britain to join US air strikes against the terrorists controlling large swathes of Iraq and Syria.
At the Nato summit in Wales, the Prime Minister insisted Britain was not yet at the stage where it was ready to take offensive military action and called for a “comprehensive plan” involving nations in the region.
Earlier, US Secretary of State John Kerry appeared to try to up the tempo, calling for agreement on an international plan for dealing with IS by the time of the general meeting United Nations General Assembly in New York later this month.
Mr Kerry and Foreign Secretary Philip Hammond jointly chaired a meeting of foreign and defence ministers from what the US called a “core coalition” of allies to discuss the way forward.
Earlier Mr Hammond said the Government will not be deterred from launching air strikes against Isil by threats to the life of Mr Haines.