President Donald Trump has postponed a decision on imposing steel and aluminum tariffs on the European Union, Canada and Mexico until June 1, a source familiar with the decision said.

The White House has also reached an agreement in principle with Argentina, Australia and Brazil, the source said on Monday, writes dailymail.co.uk.

Trump imposed a 25 percent tariff on steel imports and a 10 percent tariff on aluminum in March, but granted temporary exemptions to Canada, Mexico, Brazil, the European Union, Australia and Argentina. 

The temporary exemptions were due to expire at 12.01am on Tuesday.

If it loses its exemption, the EU has said it will retaliate with its own tariffs on US goods imported to Europe, including bourbon, blue jeans and Harley-Davidson motorcycles. 

Trump announced the tariffs on March 8, but exempted the European Union later that month.

The Trump administration has justified the tariffs - aimed primarily at Chinese overproduction - on national security grounds and told trading partners they must make concessions to win permanent exemptions.

The EU has insisted it will not negotiate without first obtaining a permanent exemption. 

In a statement released on Sunday, German Chancellor Angela Merkel said Europe was 'resolved to defend its interests within the multilateral trade framework'.

The Trump administration was also expected to announce later Monday it had reached a deal to exempt South Korea from the tariffs.

Trump is working with South Korean President Moon Jae-in ahead of the US leader's expected summit with North Korea's Kim Jong Un in the coming weeks.

Last month, the government in Seoul said it had made concessions to secure a revised trade deal with Washington and escape its steel duties.

Extensions for Canada and Mexico had been expected, as Washington, Mexico City and Ottawa work on revamping the North American Free Trade Agreement.