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.<br /><br />British Prime Minister David Cameron said the slaying of the aid worker demonstrates the necessity of destroying ISIL.<br /><br />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.<br /><br />Al Jazeera's Paul Brennan reports.
