British Prime Minister David Cameron said on Tuesday that British citizens and other nationals fighting alongside Islamist insurgents in Iraq and Syria posed the biggest threat to Britain's national security.

"No-one should be in any doubt that what we see in Syria and now in Iraq in terms of ISIS is the most serious threat to Britain's security that there is today," Cameron told a joint news conference with Chinese Premier Li Keqiang.

"The number of foreign fighters in that area, the number of foreign fighters including those from the UK who could try to return to the UK is a real threat to our country," he said.