By William Gallo

WASHINGTON - U.S. President Donald Trump says South Korea has agreed to "substantially" increase its share of the cost of the U.S. troop presence in South Korea.

"South Korea has agreed to pay substantially more money to the United States in order to defend itself from North Korea," Trump said in a tweet Wednesday.

Trump did not clarify how much more South Korea agreed to pay, but said negotiations with Seoul over cost-sharing have begun.

However, South Korea on Wednesday said talks on the issue have yet to begin.

Trump said "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."

"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!" Trump added.

The U.S. has 28,500 troops in South Korea, a remnant of the 1950s era Korean War. The Pentagon says the troops are meant to deter North Korean leader Kim Jong Un.

Trump's announcement comes a day ahead of U.S. Secretary of Defense Mark Esper's visit to Seoul. Local media had reported Esper was expected to raise the cost-sharing issue.

After months of sometimes-contentious negotiations, South Korea agreed in February to pay $925 million to support the U.S. military presence next year.

That represents an 8 percent increase from the previous year – but much less than the 50 percent spike Trump had demanded.

Since the agreement covered only one year, rather than multiple years as in the past, the issue was sure to come up again soon.

Trump routinely criticizes U.S. allies – including South Korea, Japan, and various NATO countries – for not paying enough of the cost of U.S. troops on their soil.

"It just reinforces his public disdain for U.S. allies, but it is especially poorly timed for South Korea, given its current challenges and North Korea's rolling testing," says Andrew O'Neil, a Korea specialist and professor at Australia's Griffith University.

North Korea has conducted six ballistic missile tests since May, including four in the past two weeks, after having refrained from such launches for a year and a half. It is apparently part of an effort to increase pressure on Seoul and Washington amid stalled nuclear talks.

During a recent visit to the region, White House National Security Advisor John Bolton reportedly asked both Seoul and Tokyo to increase their share fivefold, according to local media reports.

At a May political rally in Florida, Trump said that a certain country was "rich as hell and probably doesn't like us too much." The comments were widely seen as referring to South Korea.

"I won't say the country, but one country we spend a lot of money on defending – [in] very dangerous territory – and it costs us $5 billion," Trump said.

South Korea rejects Trump's view it doesn't contribute enough toward the cost of the U.S. troops, insisting it pays almost half of the total cost of $2 billion. That doesn't include the expense of rent-free land for U.S. military bases, Seoul says.

In 2017, South Korea spent 2.6 percent of its gross domestic product on military expenditures, according to World Bank data. That is a bigger percentage than any NATO member except the United States.

South Korea also paid for over 90 percent of the cost to build Camp Humphreys, the largest U.S. overseas military base, 65 kilometers south of Seoul, according to U.S. officials.