Britain's top military commander has warned that the UK is "letting down" its allies by not participating in airstrikes against Islamic State (Isil) militants in Syria.
Speaking on Sky News, chief of the defence staff, general Sir Nicholas Houghton said that the UK's policy of restricting RAF air strikes to Iraq "makes no sense" given Isil has a stronghold in Syria.
"To an extent yes, we are letting our allies down by not being a full player," he said.
"But my view on this is a far more fundamental and simple one in a way and it's the point you make about going up to a border and having to stop there."
It comes days after defence secretary Michael Fallon branded Britain's reliance on other countries to tackle Isil in Syria as "morally indefensible".
Fallon also said that he was appealing to MPs to reconsider the case for airstrikes. A government proposal for military intervention in Syria was defeated in the House of Commons over two years ago.
Houghton compared the UK's current stance to trying to win a football match, but without being able to go into the opponents' half.
"Daesh, Isil, they have a caliphate that extends across the border," Houghton said.
"But the source of their power, their command and control, their logistics, their organisation, the place from which they issue orders to international terrorists are within Syria.
"And so to be denied our ability to play a proportionate role in that makes no sense."