Jeremy Corbyn said a political settlement must be reached in Syria if we are to defeat IS as he condemned the "appalling" Paris terror attacks.
The Labour leader told ITV's Lorraine that progress had been made in talks over the weekend but more needed to be done.
"Who's funding IS? Who's arming IS? Who's providing safe havens for IS? There's some very big questions and we have to be careful.
"One war doesn't necessarily bring about peace, it often can bring yet more conflict, more mayhem and more loss," 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".