The charity which employs Yorkshire-born Islamic State hostage David Haines has said it is “deeply shocked” by the terrorist video of him broadcast this week and called for his immediate release.
ACTED said the threats made to his life were “intolerable”.
Mr Haines, 44, who is understood to have been born in Holderness, East Yorkshire, 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 French 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.”
ACTED said Mr Haines had worked as “a humanitarian” since 1999, helping people in the Balkans, 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.
The charity added: “ACTED is deeply shocked by the images broadcast earlier this week.
“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 have threatened to kill Mr Haines, who they are believed to be holding in Syria.
They have already beheaded two US 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.
In 2012 he worked in South Sudan for the Brussels-based charity Nonviolent Peaceforce, which sends unarmed civilian peacekeepers to conflict zones. In a statement it said Mr Haines acted as a “non-partisan unarmed civilian peace facilitator”.