Responding to the news the Prime Minister David 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.
For more information see ISIL: UK government response.