A week of frantic international diplomacy was under way with French President Francois Hollande set to meet all five permanent members of the UN Security Council, and the United States trying to rally support for a ceasefire in Syria.
"I firmly support the action President Hollande has taken to strike ISIL in Syria," Cameron said after a breakfast meeting in Paris, using another acronym for IS.
Meanwhile, Putin met Iran's supreme leader Ayatollah Ali Khamenei in his first trip to Tehran for eight years.
Describing the meeting as "quite constructive", a Kremlin spokesman said the two countries were against "external attempts to dictate scenarios of political settlement in Syria".
Also Read
The priority for both Iran and Russia has been to protect their ally, Syrian leader Bashar al-Assad, but Moscow has shown signs it will cooperate more closely with the West since one of its passenger jets was blown up by IS over Egypt last month.
Kerry hopes his Emirati and Saudi allies can pressure rebel factions into accepting a ceasefire with Assad within "a few weeks" -- a step seen as crucial to refocusing the war on IS.
"You can be confident that the diplomatic front is in high gear, with a very real plan on the table to be implemented," Kerry said.
In Paris, Cameron and Hollande stood side by side after laying a wreath at the Bataclan concert hall, where 90 of the 130 victims were killed in the Paris attacks of November 13.
While Britain has joined US-led coalition strikes on IS in Iraq, it has so far held back from hitting targets in Syria, where the jihadists also hold large swathes of territory.
The British leader said he had offered France the use of a strategically located British airbase in Cyprus, RAF Akrotiri, to facilitate air strikes, and assistance with refuelling French jets.
The first fighter jets took off today from France's Charles De Gaulle aircraft carrier, newly deployed to the eastern Mediterranean, for operations over IS territory in Iraq and Syria.