The dollar firmed on Monday as traders pondered the ramifications of U.S. President Donald Trump's tariff plans at the start of a week in which the Federal Reserve is widely expected to hold interest rates steady.

The dollar began the week with modest gains after a brief threat of US tariffs on Colombia made clear the trade risk that still hangs over the currency market.

Trump was quoted saying his team was thinking of tariffs around 25% which could be announced on Feb. 1 but offered no other specifics.