There were fears today for a Scot being held by the extremist group Islamic State.

The Foreign Office is investigating reports that the captive, believed to be from the Tayside area, has been threatened with death.

His plight was revealed following the release of an internet video claiming to show the beheading of a journalist by the Islamic State group.

The internet footage appears to show the death of US national Steven Sotloff at the hands of the same masked jihadi who murdered James Foley last month.

The video ends with the captor threatening to kill the Scot, who is said to be from the Dundee area.

The captive, who is not being named at the request of the Foreign Office, is also reported to have studied at Perth Academy before joining the army.

Later he moved into security and was captured last year while supporting an aid organisation.

US news channel NBC reported that he had previously worked with a civilian peacekeeping group in Africa.

An official for the group said he had experience with the military and was “very familiar with insecure locations … He was very caring, had a good sense of humour.”

The extremists’ video warned President Barack Obama that as long as US airstrikes against the militant group continue, “our knife will continue to strike the necks of your people”.

The footage – depicting what the US called a sickening act of brutality – was posted two weeks after the release of video showing the killing of James Foley and just days after Mr Sotloff’s mother pleaded for his life.

Barak Barfi, a spokesman for the family, said that the Sotloffs had seen the video but that authorities have not established its authenticity.

“The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time,” Mr Barfi said. Mr Obama boarded Air Force One last night without giving a response to the news.

Mr Cameron said in a statement: “If verified, this is a despicable and barbaric murder. My thoughts and prayers are with Mr Sotloff’s family and friends as they deal with this appalling and tragic situation.

“As I have said consistently over the last few weeks, (IS) terrorists speak for no religion.

“They threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith.

“We have already been working hard to keep British people safe and we will continue to do all we can to protect our country and our people from these barbaric terrorists.

“Tomorrow morning I will chair a Cobra meeting to review these latest developments.”

A Foreign Office spokesman said: “We are aware of a video which purports to be the murder of a US national and includes a threat to another hostage. We are urgently working to verify the authenticity of the video.”