The RAF drone strike which killed suspected British jihadis in Syria was "legally questionable", Jeremy Corbyn has claimed.
The Labour leader called for a "rapid political development" as a way to end the violence in Syria, where forces loyal to president Bashar Assad, rebels opposed to his rule and Islamic State (IS) extremists have torn the country apart.
Mr Corbyn also called for the UK to sign up to the European Union quota system to take in a share of the migrants making hazardous journeys to flee Syria and other trouble spots.
The new leader of the Opposition is expected to face a major foreign policy test in the coming months, with the Prime Minister indicating he could be prepared to seek Parliament's approval to extend the bombing campaign against IS - also known as Isis - from Iraq into Syria.
The Labour leader warned that launching air strikes "may well not be a solution" and would result in deaths and injuries.
Mr Corbyn, chairman of the Stop the War coalition, is expected to oppose any proposal to expand the air campaign against IS.
He said no one yet knew what plans Mr Cameron would bring forward, but highlighted the drone action already taken by the Prime Minister without parliamentary approval.
"I'm in no way a supporter of the Assad government or Isis, I don't think anybody believes that for one moment," he told Channel 4 News.
"But what I would say is, if we go into a bombing campaign people are going to die in that bombing campaign, people are going to be injured. There may well not be a solution to the whole problem as a result."
Setting out his preferred response, he said: "We give all the support we can to refugees at the present time, we look for a political development and solution in the whole area. We involve all the neighbouring countries - Turkey, Iran, every neighbouring country and we involve Russia in this as well, as well as the USA and the European Union and looking for a very rapid political development in the area.
"We are not there yet on what's going to happen, we don't know what the Prime Minister is going to suggest.
"What we do know is that he authorised drone attacks, which is legally questionable."
Asked if the UK should be prepared to take any quota of migrants recommended by Brussels, he said: "Absolutely and I think it is a great mistake that Britain didn't join the European Union refugee programme when it should have done three years ago, because we have a responsibility - just like every other country - under the Geneva Convention of 1951 to take people who are victims of oppression, war and all those awful things and we should take our share."
The drone strike which killed Britons Reyaad Khan and Ruhul Amin in the IS stronghold of Raqqah was justified on the grounds of "self-defence", David Cameron told MPs earlier this month.
Attorney general Jeremy Wright had agreed there was a "clear legal basis", Mr Cameron said.