WASHINGTON ― Within days of multiple North Korean missile tests, U.S. President Donald Trump has said “talks have begun” for South Korea to pay the U.S. more for its defense.

In a pair of posts to Twitter Wednesday morning, Trump argued the U.S. pays too much to cover the Pacific ally’s defense costs. He said Seoul pays the U.S. $990 million and it should pay "substantially more money to the United States.”

“Talks have begun to further increase payments to the United States,” Trump wrote. “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!”

South Korea has agreed to pay substantially more money to the United States in order to defend itself from North Korea. 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.. — Donald J. Trump (@realDonaldTrump) August 7, 2019

South Korea’s Foreign Ministry said Wednesday that the cost-sharing talks had not officially started. But it said that U.S. and South Korean officials during Trump national security adviser John Bolton’s visit to Seoul last month agreed that the upcoming negotiations should proceed in a “rational and fair direction.”

Trump’s comments come as Kim Jong Un’s regime in North Korea has been test-firing missiles it says are a warning over U.S.-South Korean military exercises.

Also, a stopgap agreement for South Korea to increase its payments from $850 million in 2018 to $924 million in 2019 is set to expire in December, according to an analysis by the Congressional Research Service.

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Trump’s financial demands have triggered worries in South Korea that he might withdraw some of the 28,500 U.S. troops.

In addition to the presence of U.S. troops, South Korea is included under the U.S. “nuclear umbrella,” and traditionally has paid for about 50 percent, or more than $800 million annually, of the total non-personnel costs of the U.S. troop presence in South Korea, according to an analysis by the Congressional Research Service.

After several years of close coordination, notably on North Korea, collaboration between Washington and Seoul became more inconsistent and unpredictable under the administrations of Donald Trump and Moon Jae-in, according to the analysts. Moon, a progressive, was elected in May 2017 after a decade of conservative rule in South Korea.