David Cameron has been urged to spell out the details of any military intervention he is considering in Syria, as French President Francois Hollande announced he is preparing for air strikes.
Mr Hollande told a news conference in Paris that France will start reconnaissance flights on Tuesday with a view to launching attacks on Islamic State militants, but ruled out sending troops on the ground.
The Prime Minister said on Friday that he would only seek Parliament's approval to extend RAF bombing operations against IS - also known as Isil or Isis - into neighbouring Syria if he was confident there was a "genuine consensus".
He has been wary of a repeat of his 2013 defeat when military action against Syrian dictator Bashar Assad was rejected by MPs.
But he and Chancellor George Osborne have made clear that they see a strong case for air strikes within Syria, and expectations are high of a Commons vote soon after the September 12 election of a new Labour leader.
Mr Osborne hinted on Sunday that even opposition from the incoming Labour leader - widely expected to be left-winger Jeremy Corbyn - may not prevent the PM seeking parliamentary authorisation for action.
"We need to see support across the House of Commons for this action," said the Chancellor. "It doesn't mean that everyone has to sign up to it. We have got to spend the coming period making that argument to people."
Cameron welcomes announcement
Asked whether Mr Cameron welcomed Mr Hollande's announcement, the PM's official spokeswoman said: "The Prime Minister has repeated recently that he thinks there is a strong case for taking action against Isil in Syria, as in Iraq. It is important that this is a coalition effort.
"This is one of the biggest challenges we face and we should be doing all we can to defeat Isil."
She added: "He does think - and both he and the Chancellor have been saying - that we need to look at what more can be done to solve the conflict in Syria."
Pressed over whether Mr Cameron was planning a Commons vote on extending air strikes, she said: "The Prime Minister made clear on Friday that he would have to be confident that there had been a change of view in Parliament before he took the issue back to Parliament."
Labour leadership candidate Yvette Cooper called on the Government to offer clarity on how UK military action could help in Syria.
Ms Cooper told the BBC Radio 4 Today programme: "I think with something as serious as military intervention you have to look at what the Government is proposing.
"They have to be clear what it is they want to do that will help. In Syria you have the problem with both Isil and Assad.
"In Syria, what the whole Government needs to explain, do they want to help Assad?"
She added: "Asking an opposition to respond without seeing what the clear proposals are would not be responsible."
Meanwhile, former international development secretary Andrew Mitchell called on the UN to establish two safe havens near Syria's borders with Turkey and Jordan to protect millions of refugees.
"Ideally Britain would not be involved in putting troops on the ground but we should be willing to consider that," the Tory MP said.
"We would be talking about an international body hopefully composed of troops from Jordan and Egypt and other countries in the region willing to shoulder the burden."
But Mr Cameron's spokeswoman said that any such scheme would face the challenge of "how you make it work to ensure it truly is a safe haven for people".