Home Secretary Sajid Javid has warned he could use new powers to ban British nationals from Syria.
A new counter-terrorism act allows him to ban people from travelling to, or remaining in, certain areas – or face up to 10 years in prison.
He said Britons in Syria without good reason were “on notice”.
Mr Javid also announced plans for a new Espionage Bill with “new and modernised” powers to tackle hostile state activity.
The bill may include a requirement for spies in the UK to register with the government so that it is “easier for us to act if there is hostile state activity”.
Mr Javid also appointed Jonathan Hall QC as a watchdog to monitor the new terror laws.
The decision to ban travel to Syria comes after Britons who joined the Islamic State group sought to return to the UK.
Speaking to senior security figures in central London, Mr Javid set out for the first time how he expects to use the new Counter-Terrorism and Border Security Act.
He said: “I’ve asked my officials to work closely with the police and intelligence agencies to urgently review the case for exercising this power in relation to Syria, with a particular focus on Idlib and the north east.
“So anyone who is in these areas without a legitimate reason should be on notice.”
‘Long time coming’
Sir Peter Fahy, former counter-terrorism lead for the Association of Chief Police Officers, told BBC Radio 4’s Today programme ahead of Mr Javid’s speech: “It’s been a long time coming. Clearly the conflict in Syria has been going for many years and there’s been this problem of what to do with people who go overseas.
He said the legislation was welcome but there would be complications over who it covered.
The north-western Syrian province of Idlib is the last remaining stronghold controlled by forces opposed to President Bashar al-Assad.
Mr Javid said police and security services “have worked tirelessly” to identify people intending to join the Islamic State group overseas and prevent them from leaving the country.
He told the House of Commons in February that 900 people people from the UK were estimated to have joined the Islamic State group in Syria and Iraq.
Of those, 40% were estimated to still be somewhere in the region, 40% to have returned, and 20% to have been killed in battle.
The issue came to the fore following the case of Shamima Begum – who had her UK citizenship revoked by Mr Javid in February, after she was found in a Syrian refugee camp.
Ms Begum left London to join the Islamic State group when she was 15.
Mr Javid also emphasised the importance of international co-operation in combating terrorism.
“As these threats become more global we all rely on an international system of defence, policing, security and intelligence – a safety net based upon co-operation and unity,” he said.
“These structures rely upon free, democratic nations to pool information, coordinate law enforcement action and surrender suspected criminals across borders.
“More than any other country on Earth, the UK has a coherent, connected approach to intelligence and security and when threats appear, the world still turns to the UK for leadership, support, and action.”
Sir Peter agreed there was a “tremendous need” to reassure allies in such a way, adding: “The world is a very uncertain place at the moment… the whole issue about Brexit, this issue about Huawei and the situation with Iran is creating tension with the United States.
“People involved in counter-terrorism will be looking to see if that does affect the level of co-operation.”