South Korea has long come under targeted criticism from Trump, who has called it a "money machine" that takes advantage of American military protection.

The two leaders held three meetings during Mr Trump's first term in office and exchanged a number of, what the president called, "beautiful" letters.

North Korean leader Kim Jong Un has supervised the test-firing of two types of new antiair missiles displaying his expanding military capabilities as the South Korean and U.S. militaries carry out joint drills.