The ISIS threat to behead British hostage David Haines couldn't have been more impeccably timed for David Cameron, arriving less than 48 hours before a speech by the British Prime Minister in which he began to build a case for air strikes targeting Syria.

Haines appeared at the end of a video released on Tuesday which purported to show the beheading of American journalist Steven Sotloff. The ISIS militant in the footage threatens to kill Haines if the US and the UK do not "back off" from fighting ISIS.

At the start of the NATO summit in Wales this morning, Cameron cited the video as one of the reasons why air strikes on Syria cannot be ruled out, as he began re-building a case for military aggression that failed last year following dubious claims about who was responsible for the Ghouta chemical weapons attack.

For Cameron, who has been struggling to build political support for air strikes on ISIS, the threat to behead Haines couldn't have been more timely.