Washington DC - The US will require Canadians visiting for more than 30 days to register with authorities, the federal register showed Wednesday, toughening rules as trade tensions soar.

The new requirement, effective from April 11, would harden enforcement of an existing law from which US media said Canadian nationals had typically been exempt.

It will likely impact the estimated 900,000 Canadians – known colloquially as "snowbirds" – who spend winters in warmer southern US states such as Florida, Texas, and Southern Carolina.

The move marks a further deterioration in ties between the historical allies since President Donald Trump took office in January.

Canada Canada says tariffs on US auto imports come into force Wednesday

Fresh US tariffs of 25% came into effect Wednesday on steel and aluminum – widely seen as a blow to Canada, which already faces a separate 25% levy on other goods.

Trump has, meanwhile, repeatedly called for Canada to become the 51st US state – a taunt seen by some north of the border as an annexation threat.

The Department of Homeland Security estimates that between 2.2 million and 3.2 million people will be impacted by the new visitor registration rules.