British Prime Minister David Cameron says a Royal Air Force drone strike has killed three Islamic State fighters in Syria including at least one British man who had been planning attacks against the U.K.

Cameron told Parliament on Monday that the attack was lawful, saying the attorney general had been consulted and had approved the attack.

The prime minister said the threat from Islamic State fighters was more acute than ever before and that the drone attack was the only "feasible means" of dealing with the danger.

Cameron also said police and security forces had thwarted at least six terror attacks inside Britain in the last 12 months.