Shortly after Trump's tweet, Seoul's foreign ministry said that official negotiations on the Special Measures Agreement (SMA), a bilateral deal to share the cost of stationing 28,500-strong US Forces Korea, have yet to begin.

"It is inappropriate to comment on remarks on the social media of a foreign head of state," the ministry said in a statement. "The 11th SMA negotiations have yet to be officially launched."

The ministry also pointed out that Seoul and Washington agreed to conduct discussions on the new SMA in a "reasonable and fair" way when US National Security Adviser John Bolton visited Seoul last month.

"(The two sides) plan to discuss concrete details (of the cost sharing) in the 11th SMA negotiations," the ministry said.

"(South Korea) is seeking to pick its new chief negotiator and form a task force (for the negotiations) through an internal government review," it added.

The allies are expected to hold the 11th SMA talks in the coming months, as the current one-year deal is set to expire on Dec. 31.

Under the 10th SMA, struck in February, Seoul agreed to pay 1.04 trillion won ($879 million) this year, an increase of 8.2 percent from 960 billion won last year.

Washington is expected to demand a hefty rise in Seoul's contribution, given that Trump has pressured American allies to jack up their defense costs.

Since 1991, Seoul has shouldered partial costs under the SMA -- for Korean civilians hired by the USFK, the construction of military facilities to maintain the allies' readiness and other forms of support.

Speaking to reporters at the White House later in the day, Trump again claimed that he and South Korea "have made a deal."

"They've agreed to pay a lot more money toward the United States," he said. "As you know, we've got 32,000 soldiers on South Korean soil, and we've been helping them for about 82 years. And we get nothing. We get virtually nothing."

Trump said that under the new deal South Korea will pay "a lot more money."

The relationship, he said, is very good.

"But I felt, all along, I felt for years it was a very unfair one," the president said. "So they've agreed to pay a lot more, and they will agree to pay a lot more than that. And we're with them. We're with them." (Yonhap)