The UK government will introduce legislation to tackle the phenomena of Britons joining militant groups in Iraq and Syria, including the seizure of passports of suspected militants, Prime Minister David Cameron said on Monday.

"We will introduce specific and targeted legislation ... providing the police with a temporary power to seize a passport at the border during which time they will be able to investigate the individual concerned," Cameron told parliament, announcing measures designed to tackle the threat of radicalized Britons fighting alongside the Islamic State of Iraq and Syria (ISIS)

Cameron insisted the plans would not represent “sweeping new powers” and that he wished to avoid “knee-jerk reactions.”

The proposals come days after Cameron raised Britain’s terrorism alert to “severe,” its second-highest level, saying ISIS posed the country's greatest-ever security risk.

The move, which means an attack is considered "highly likely,” came after the killing of U.S. journalist James Foley. Cameron has said that it seemed "increasingly likely" that a British militant was behind Foley’s execution.

About 500 British citizens are believed to have gone to Syria to join militant groups such as ISIS.

(Al Arabiya with Reuters, AFP)