LONDON - Britain has killed two of its nationals in Syria, suspected of fighting for Islamic State (IS), with a drone strike, Prime Minister David Cameron said on Monday, despite the government not having a parliamentary mandate to take military action in the country.

Britain conducts regular attacks in neighbouring Iraq and flies drones over Syria to gather intelligence on the hardline IS group. But unlike some other coalition partners it does not target IS positions in Syria as Cameron failed to get the go-ahead from parliament to do so in 2013.

Cameron told parliament on Monday that, as an act of self defence, one Briton had been killed in a precision airstrike carried out by an RAF remotely piloted aircraft in August. Two others travelling with the man - including another Briton - were also killed.

"We took this action because there was no alternative," he said, adding that the men had been plotting attacks against Britain.