David Cameron has given the French Air Force the use of Britain's RAF base in Cyprus as he promised the UK would do "all in our power" in the war against ISIS.
Speaking alongside Francois Hollande after talks in Paris, Cameron said he firmly supported the French President's strikes on Syria and added: "It is my firm conviction that Britain should do too."
The Prime Minister announced the countries would increase counter-terrorism cooperation and called for an improvement in EU border checks and the sharing of airline information.
"The United Kingdom will do all in our power to support our friend and ally France to defeat this evil death cult," Cameron said.
"It is clear that the world is coming together to tackle this evil terrorist threat. We have shown our firm resolve and together we will defeat this evil threat."
Ahead of the joint press conference, Cameron stood "shoulder to shoulder" with the French President at the concert hall where 89 people were murdered by Islamic State terrorists.
The Prime Minister is likely to use the discussions with Hollande to help persuade MPs to back a vote to expand the UK's military role.
On Thursday he will present his blueprint for Britain's part in the war on ISIS in response to the findings of the influential Commons Foreign Affairs Committee, which warned against military involvement until there was an international strategy to end the conflict.
Hollande said he and Cameron had been working on a "political solution" for Syria for three years and said both were convinced the country could not continue to be run by Bashar Assad but needed a "constitutional government".
The French President, who is pushing for a "grand coalition" to defeat ISIS, said: "We are convinced that we must continue to hit Daesh (ISIS) in Syria.
"We will intensify our strikes, we will cause the greatest damage possible to this terrorist army.
"Our air forces have been told to strike and strike hard against Daesh."
French fighter jets took off Monday from the Charles De Gaulle aircraft carrier in an operation against ISIS in Iraq and Syria, military sources said. However, no further information about the operation was given.
Hollande will meet with Barack Obama, Vladimir Putin and Angela Merkel later this week.
The United Nations Security Council has backed a French-drafted resolution urging countries to "combat by all means" the "unprecedented threat" of ISIS.
Cameron said the vote "shows beyond doubt the breadth of international support for doing more in Syria and for decisive action to eradicate" ISIS.
The Prime Minister has been seeking to win the support of MPs for strikes in Syria with a series of briefings.
He has said he will not put the matter to the vote in the House of Commons until he is certain of victory because it would damage the UK's global reputation.
Speaking to Sky News, Defence Secretary Michael Fallon said the Government was briefing MPs.
"We have to get out and convince, particularly new MPs, of the case," he said.
It is understood that the first bombing raids against Islamic State would begin within hours of a "yes" vote.
Labour leader Jeremy Corbyn has made clear his opposition to strikes in Syria but on Monday shadow defense secretary Maria Eagle said it was "her understanding" Corbyn was "not a pacifist" and it was "conceivable" he could vote for action.
Speaking on the Radio 4 Today programme, she said Cameron could win some support from Labour MPs if he made a compelling case.
Speaking to Sky News last week, Corbyn suggested that Labour MPs would not be given a free vote on Syria, however, at the weekend the shadow chancellor John McDonnell said they should be allowed to vote on conscience - putting the two men at odds.
Corbyn has come under fire from his own MPs over his reaction to the Paris attacks.