British nationals spending time in Syria face arrest and up to 10 years in jail on their return home in new anti-terrorism legislation passed by parliament, AFP reported Wednesday (February 13th).

Intelligence services estimate there are hundreds of British fighters still in Syria, with the territorial demise of the "Islamic State of Iraq and Syria" (ISIS) bringing their fate into sharp focus.

The new British law toughens previous legislation that required authorities to prove returning nationals had engaged in terrorist activities while abroad.

Interior minister Sajid Javid will now have the power to declare any zone outside Britain a "designated area" in order to "protect members of the public from a risk of terrorism".

Just visiting such zones will now constitute a crime, with exceptions for those with a "reasonable excuse for entering, or remaining in, the designated area".

They include people providing humanitarian aid, armed forces, UN staff, journalists and those attending funerals or visiting ill relatives.

Britons will have a month to leave the area after the law is enacted, which is likely to be within a few months.

British authorities had found it difficult to prosecute those coming back to Britain due to the challenges in securing evidence of them committing individual acts.

Javid said the new legislation would make it easier to "punish those who seek to do us harm".

Meanwhile, defence minister Gavin Williamson said Wednesday that Britain is ready to do "all that is required" to neutralise the threat from ISIS, after the US suggested creating a new international mission in north-east Syria.