"Tariffs are actually, we've had a lot of experience with them. They're an act of war, to some degree," Buffett said.

Trump's Commerce secretary said in a CNN interview that Canada and Mexico still need to do more to stop fentanyl flows into the U.S.

Mexico will wait and see if U.S. President Donald Trump goes through with his threat to slap tariffs on its southern neighbor on Tuesday, the nation's president said on Monday, adding that Mexico has back-up plans in case the tariffs go into place.