U.S. President Donald Trump said on Monday he had rejected a sum offered by South Korea in response to his demand that Seoul pay for a larger share of the cost of U.S. military forces deployed there.

"Now they've offered us a certain amount of money and I've rejected it," Trump said at a White House news conference, adding that Seoul was paying about a billion dollars a year towards a U.S. troop presence of 28,000 to 32,000 personnel.

"We're defending a wonderful nation. We're asking them to pay for a big percentage of what we're doing. It's not fair. ... It's a question of will they contribute toward the defense of their own nation," Trump added.