Two major European countries are warning citizens about travel to the U.S. following high-profile cases in which American federal immigration authorities detained tourists and U.S. green cardholders trying to enter the country.

Britain has updated its travel advice for the US, warning visitors they could face ‘arrest or detention’. The US ‘enforces entry rules strictly’ and those who do not abide by them could be detained, the Foreign Office says. ‘You should comply with all entry, visa and other conditions of entry,’ the department’s new advice reads.

As the U.S. tightens immigration enforcement under President Donald Trump, both the United Kingdom and Germany have updated their travel advisories to warn citizens that holding a valid visa or ESTA authorization does not guarantee entry into the country.