British Prime Minister David Cameron said on Friday he planned to introduce new legislation to make it harder for Britons to travel to Syria and Iraq to fight alongside Islamist extremists.

Cameron, who said he would detail his plans in parliament on Monday to confiscate passports, was speaking as Britain raised its international terrorism threat level to "severe", its second highest level.

"I will be making a statement in the House of Commons on Monday. This will include further steps to stop people travelling, with new legislation that will make it easier to take people's passports away," Cameron told a news conference.