The family of a British aid worker believed to be held by Islamic militants in Syria has issued a public plea urging his captors to contact them.

Extremists from the Islamic State group have threatened to kill 44-year-old David Haines, and his family said they have not received any reply from the group after repeated messages.

In a short statement released through Britain's Foreign and Commonwealth Office late Friday, the family said: "We are asking those holding David to make contact with us."

Islamic State militants have beheaded two American journalists and posted the evidence online in videos. At the end of the last video, they threaten to kill Haines next.

Foreign Secretary Philip Hammond has said the British government is doing everything it can to protect Haines.