David Cameron insists 'brutal' Islamic State captors are solely responsible for Kayla Mueller's demise
David Cameron has said he is "deeply saddened" by the death of US aid worker Kayla Mueller - and insisted her "brutal" Islamic State captors are solely responsible for her demise.
The Prime Minister added his voice to tributes after US president Barack Obama confirmed that the parents of the 26-year-old, from Arizona, have been told she has been killed.
Islamic State - also known as Isis or Isil - has claimed she died in a Jordanian air strike, but the authorities in Amman have dismissed the assertion as propaganda.
Mr Cameron said: "I am deeply saddened to hear of the death of US aid worker Kayla Mueller and my thoughts are with her friends and family as they deal with this awful news.
"Let me be clear - those responsible for her death are the terrorists that kidnapped Ms Mueller and who held her against her will ever since. Kayla travelled to the region to help men, women and children forced to flee their homes and her death demonstrates yet again the indiscriminate brutality of Isil. We will work with coalition partners to ensure her murderers face justice, however long it takes.
"The truth is that Isil are under increasing pressure from the coalition of over 60 nations, including those with large Muslim communities, who are giving help and support to local forces to take back territory in Iraq and Syria. Despite its attempt to break this effort, Isil's depravity has only strengthened the international coalition against them."
Ms Mueller is believed to be the fourth American to die while being held by Islamic State, and was the only known remaining US hostage known to be in the hands of the group. She was taken in August 2013 while leaving a hospital in Syria.
Parents Carl and Marsha Mueller said in a statement: "We are heartbroken to share that we've received confirmation that Kayla Jean Mueller has lost her life.
"Kayla was a compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice and peace."
Foreign Secretary Philip Hammond said the terror group's violence "will not succeed".
In a tweet he said: "Deeply distressing to hear of death of aid worker Kayla Mueller. My thoughts with her family and friends. #ISIL violence will not succeed."