"This is an important moment," said Cameron in a statement soon afterwards.
"The international community has come together and has resolved to defeat this evil, which threatens people of every country and every religion."
Britain is already involved in air strikes against Islamic State militants in Iraq.
Cameron wants to extend the operation to hit Islamic State in Syria to fall into line with allies, and has said he will submit a plan to parliament to do so.
Islamic State has seized large areas of territory in Syria and neighbouring Iraq. A U.S.-led coalition has been bombing the militants for more than a year and Russia started air strikes in Syria in September.
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