Labour leader Jeremy Corbyn has indicated he believes the killing of Isis fighter Mohammed Emwazi may not have been legal under international law.
He told ITV News correspondent Chris Ship that he "questioned" whether the killing of Emwazi was legal.
"I'm awaiting an explanation of where the legal basis was for that incident," he said.
Mr Corybn also said he did not believe Prime Minister David Cameron would be able to secure a majority vote in parliament in support of air strikes on Syria and that there would be no free vote for Labour MPs on the issue.
"I have certainly not agreed to a free vote," he said.
David Cameron has revealed he will "make the case" for Britain to join its allies in tackling Islamic State in Syria, instead of just Iraq.
The Chancellor has said the government will only call a vote on UK military action in Syria when it is "confident that we are going to win".
Labour's shadow chancellor has said the notion of any UK intervention in the Syria conflict has to be looked at in a "very considered way".