The British Government could ban travel to parts of Syria and West Africa in a bid to combat the problem of foreign fighters who join extremist organisations such as Isis (Islamic State).
Home Secretary Sajid Javid said that he was asking counterterrorism officials to look into whether it would be appropriate to use the authority vested under a recently enacted law "in relation to Syria, with a particular focus on Idlib and the northeast."
Speaking at New Scotland Yard, the headquarters of London's Metropolitan Police Service, Javid suggested that British citizens who are in those parts of Syria should consider themselves "on notice". He also said that the ban may not be limited to Syria.
"I can also see that there may be a case in the future for considering designating parts of West Africa," Javid added.
The proposal comes just months after the collapse of Isis' self-proclaimed "caliphate" in Syria and Iraq and several years after governments in Europe and North America began publicly raising awareness of the problems posed by their citizens travelling to fight alongside the group.
An estimated 900 British citizens are thought to have travelled to Syria or Iraq to join groups like Isis in recent years, with some taking on prominent roles in disseminating propaganda and carrying out killings.
The British proposal suggests that the Government is also worried about foreign fighters heading to other parts of the world.
A number of countries in West Africa are battling Islamist extremist groups, including Nigeria, which has been fighting the Isis-affiliated Boko Haram for years. To date, these conflicts are not believed to have attracted significant numbers of fighters from Britain or other Western nations.
"There are dozens and dozens of areas where terror groups are fighting, and at some point they could be attractive for foreign fighters," said Phil Gurski, a Canadian terrorism expert and former analyst with the Canadian Security Intelligence Service.
"The problem is, once you open this door, you can make a case for banning travel to about half the world," Gurski said. "It becomes a game of whack-a-mole in reverse."
A government ban on travel to a specific region because of terrorism fears is unusual. The US has banned its citizens from travelling to Lebanon, Iraq and Libya in the past, and it currently bans travel to North Korea, but citizens' safety was the motivating factor.
In 2014, Australia similarly banned travel to Iraq and Syria, though the law has been enforced relatively infrequently.
David Malet, a professor at American University, says the US and some other countries generally already have broad rules that prohibit offering material support to terrorist groups. "A woman who they say cooks for her husband? That could be material support," Malet said.
The powers that Javid outlined were included in the Counterterrorism and Border Security Act 2019. Under that law, the British Home Secretary has the power to recommend that British citizens and residents be restricted from travelling to an area; the recommendation would require Parliament's approval.
The punishment for such travel would be up to 10 years in prison, according to the law.
Since the collapse of the caliphate earlier this year, a debate has raged in Britain over how to deal with captured British citizens who had supported Isis. Javid has favoured tougher measures, including revoking the British citizenship of Shamima Begum, a Londoner who joined the group when she was 15.
Such measures may have only a limited deterrence value. "It seems like it's really hard to deter anyone from becoming a foreign fighter," Malet said. "They are almost always breaking the law when they travel."
Malet said that in Australia's case, the law has been used more to deter Australian Isis members who were thinking about coming back.
Some countries, including Canada, have been more proactive, taking steps to deter potential terrorism recruits from travelling, including blocking them from leaving the country. But that can carry its own risks: Two deadly incidents that occurred just days apart in Canada in 2014 involved Isis supporters whose travel had been blocked.
"If you pass a law that says thou shalt not go, people might say, 'If I can't do it there, I'll do it here,' " Gurski said. "Be careful what you wish for."