The US has issued a worldwide travel alert to its citizens, citing terrorist threats from Isis, al-Qaeda, Boko Haram and "other terrorist groups".
The alert from the US State Department urges US citizens to exercise caution in public places and during holidays and public events.
French President Francois Hollande will meet Barack Obama at the White House on Tuesday to discuss the fight against so-called Islamic State.
David Cameron and Francois Hollande have visited the Bataclan concert hall where 89 people were killed during deadly terror attacks in Paris.
"President Hollande and I stood shoulder to shoulder outside the Bataclan Cafe in Paris," the prime minister tweeted.
France and the US have agreed to step up airstrikes in Syria and Iraq to target IS in the wake of the Paris attacks.
Britain will do "all in our power" to support France in the fight against IS following the Paris terror attacks, David Cameron said.
The United Nations Security Council agreeS to intensify efforts to 'eradicate' Islamic State in the wake of the Paris terror attacks.