Image copyright Handout Image caption Mr Kassig was abducted at an IS checkpoint in Syria

An American medical aid worker has been threatened by an Islamic State militant in a video that appears to show the beheading of a UK hostage.

Peter Kassig is shown at the end of a video apparently showing the death of British taxi driver Alan Henning, and a masked man says he will be next.

The 26-year-old from Indiana has worked in Lebanon, Turkey and Syria to help victims of the Syrian conflict.

The BBC understands he was kidnapped when in an ambulance in Syria in 2013.

Mr Kassig's capture at the hands of IS was confirmed by the White House on Friday evening.

"At this point we have no reason to doubt the authenticity of the video released earlier today," said spokesperson Caitlin Hayden.

"We will continue to use every tool at our disposal - military, diplomatic, law enforcement and intelligence - to try to bring Peter home to his family."

Image copyright Handout Image caption Kassig delivered food and medical supplies to people caught up in the Syria war

A family statement referred to him as Abdul-Rahman Kassig, saying he had converted to Islam while in captivity. And they extended their concern for the family of Alan Henning.

"We have read about his work and his generous character with great respect and admiration.

"We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe."

A US Army veteran, Mr Kassig was discharged after the Iraq War and he became an emergency medical technician.

He travelled to Lebanon in May 2012 to work as a volunteer helping Palestinian refugees and as time went on, people affected by the war in Syria, setting up a charity called Special Emergency Response and Assistance (Sera).

In an interview before his capture, Mr Kassig described himself as an idealist who had found a sense of purpose in aid work amidst the complexities of the Middle East conflict.

Image copyright Reuters Image caption Henning was described as a British hero by the UK Foreign Office

The White House condemned the "brutal murder" of Mr Henning, who it had said "worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom".

Mr Henning becomes the fourth Western hostage to be killed by the group.

The beheadings of US journalists James Foley and Steven Sotloff, and Scottish aid worker David Haines, have previously been filmed.

The father of another UK hostage, John Cantlie, appealed on Friday for him to be released.

His son, a journalist, was kidnapped in Syria in 2012 and has so far appeared in three videos.

Islamic State (IS) - also known as Isis or Isil - has taken control of large areas of Syria and Iraq and declared a caliphate.

The US has led a campaign of air strikes in Iraq for several weeks and in Syria for almost two weeks.