"Over the past many decades, the U.S. has been paid very little by South Korea, but last year, at the request of President Trump, South Korea paid $990,000,000," Trump posted on Twitter.

"Talks have begun to further increase payments to the United States. South Korea is a very wealthy nation that now feels an obligation to contribute to the military defense provided by the United States of America. The relationship between the two countries is a very good one!"

The two countries have been in a security alliance since the 1950-53 Korean war, which ended with an armistice rather than a peace treaty -- with more than 28,000 US troops stationed in the South to guard against threats from Pyongyang.

But Trump has repeatedly complained about the expense of keeping US forces on the peninsula, and Seoul said in February it had agreed to hike its payment for maintaining American troops on its soil from $850 million to $924 million in 2019 -- 8.2 percent more than what it offered under a previous five-year pact which expired at the end of last year.

The row had raised concern that Trump might use it as an excuse to withdraw US troops.

Pyongyang on Tuesday fired two projectiles that "are assumed to be short-range ballistic missiles" into the sea, the fourth pair fired in less than two weeks, and the North has threatened more.

Kim says the nuclear-armed country's latest missile launches were a warning to Washington and Seoul over their joint war games, state news agency KCNA reported on Wednesday.

The South Korean and US militaries began mainly computer-simulated joint exercises on Monday to test Seoul's ability to take operational control in wartime.

Those drills are taking place despite Pyongyang's warnings that the exercises would jeopardize nuclear negotiations between the United States and North Korea.