World leaders have condemned the beheading of British hostage Alan Henning by the Islamic State of Iraq and the Levant, after the fourth such video was released on the Internet.

British Prime Minister David Cameron said the slaying of the aid worker demonstrates the necessity of destroying ISIL.

An international coalition of led by the US has been carrying out air strikes in Iraq and Syria to stop thousands of fighters from gaining more ground.

Al Jazeera's Paul Brennan reports.