david haines
British aid worker David Haines shown in a video released by Isis.YouTube

The family of British aid worker David Haines, who is being held hostage by Isis (Islamic State) militants in Syria, has appealed to the terrorists to contact them.

A statement released by the Foreign and Commonwealth Office, on behalf of the family, said: "We are the family of David Haines. We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us."

Haines, a 44-year-old father-of-two, was captured by the Islamic State in Syria in March 2013 while working for the Agency for Technological Cooperation and Development (Acted).

He was abducted along with an Italian colleague while travelling in a car through northern Syria on the way to the Turkish border.

Execution threat

Haines' captors have threatened to kill him if world leaders do not agree to their demands, and have yet to respond to any of the family's attempts to establish contact.

Islamic State militants beheaded American journalists James Foley and Steven Sotloff in August and September, and posted videos showing the killings online.

Following Sotloff's murder, Haines was paraded in front of cameras by a masked British militant known as "Jihadi John", who vowed to kill him next if the US and Western allies do not "back off" from their operations against the militant group.

Haines has worked in humanitarian aid since 1999 and has provided assistance to victims of conflict in Africa, the Balkans and the Middle East.

Aid workers 'should never be threatened'

During Libya's 2011 civil war, Haines worked in the country as head of mission for Handicap International, which helps disabled people in conflict zones and poor areas around the world.

Before his capture, Haines was living in Croatia with his second wife Dragana, with whom he has a four-year-old daughter. Haines also has a 17-year-old daughter with his first wife Louise.

"He's everything to us," Haines' wife Dragana told The Telegraph. "He's our life. He's a fantastic man and father. Nobody can understand how we are feeling."

UK foreign secretary Philip Hammond said the Foreign and Commonwealth Office will do everything it can to protect Haines.

Haines' employer Agency for Technical Cooperation and Development (Acted) joined the calls for his release last week, saying in a statement: "A man's life should never be threatened on account of his humanitarian commitment."