The UK will take "whatever steps are necessary" to keep safe after a video showing the killing of hostage David Haines was issued by Islamic State militants, the prime minister has said.

David Cameron said the UK would "hunt down" the killers of the aid worker, whom he called a "British hero".

He said the "menace" of IS had to be destroyed in a "calm, deliberate" way.

In the video, IS also threatened to kill a second Briton, who has been named as aid worker Alan Henning, 47.

Mr Haines was seized in Syria in 2013. He was being held by Islamic State militants who had already killed two US captives, and a video of his death came shortly after his family appealed to his captors to make contact with them.

Born in Holderness, East Yorkshire, Mr Haines went to school in Perth and had been living in Croatia with his second wife, who is Croatian, and their four-year-old daughter. His parents live in Ayr.

The video of the 44-year-old's beheading was released on Saturday night.

A masked man who appears to have a British accent was pictured beside Mr Haines holding a knife.

Mr Henning, a married father-of-two from Salford, worked as a taxi driver.

Catrin Nye, from the BBC's Asian Network, met Mr Henning in the UK while filming documentaries about British aid convoys to Syria.

She said he had been inspired by friends who had been on the convoys and was preparing for his second visit to the country.

"He described the fact that he had been before, he had visited a refugee camp... and had a really quite life-changing experience," she told the BBC News Channel.

"It had really touched him and he said ever since he had got back things hadn't quite been the same and he felt a real desire to go again and to help the Syrian people."

US Secretary of State John Kerry has completed a tour of the Middle East, aimed at enlisting allies against IS, which has also killed two American journalists.

Several Arab countries have offered to take part in air strikes against IS militants in Iraq, US officials say.

BBC