Canada is due to announce its answer to US President Donald Trump's latest wave of tariffs Tuesday, as a trade war intensifies between the historically close allies.

The new measures aim to support workers and businesses during "challenging times," Canada's government said late Monday, with officials to hold a press briefing to detail Ottawa's response Tuesday morning.

Fresh 50-per cent US tariffs took effect on a range of Canadian goods Saturday, after trade negotiations collapsed the prior day at the eleventh hour.

The duties impact some $20 billion in goods, or about 5.5 per cent of Canadian exports to the United States, economists estimate.