LONDON (AP) -- Speaking to Parliament, Cameron also said his government is working on plans to block those suspected British jihadi fighters from re-entering the U.K.
The government would also get stronger powers to monitor those suspects who are already in Britain.
The plans to widen the country's anti-terror laws are aimed at preventing attacks by Islamist militants returning from terror training in trouble spots in the Middle East. The plans are expected to be approved by Parliament.
Intelligence and security services suspect that around 500 Britons have gone to fight in Syria and potentially Iraq. Cameron has described the extremism posed by the Islamic State group as the biggest security threat of modern times -- surpassing that of al-Qaida -- and said it poses a direct threat to Europe.