"The strong and essential partnership between our countries has never been more important...we are continuing the fight against Daesh (ISIS) in Iraq and Syria," Cameron told reporters at a joint press conference at Downing Street after his bilateral talks with the US President.
Obama arrived for his three-day visit of the UK late on Thursday night.
On a personal note, he said he had come to the UK to wish the Queen a happy 90th birthday, describing her as a "jewel to the world".
"She is truly an astonishing person If we are fortunate to reach 90, we can only hope to be as vibrant as she is," he said.
After his meeting with Cameron, Prince William and Kate hosted him and US First Lady Michelle Obama for an informal dinner at their Kensington Palace home in London.
Obama's UK stay is part of a tour covering Saudi Arabia, which he left on Thursday after having discussions with King Salman, as well as Germany after the UK.