US President Donald Trump said Friday that he received a "beautiful" letter from North Korea's leader this week in which Kim Jong-un expressed his displeasure with military exercises between South Korea and the US.

Speaking to reporters at the White House, Trump said he was never a fan of the exercises either but he let them happen this time because they allowed a "turnover of various areas" to South Korea, which he liked.

The allied drills began Monday to test South Korea's capabilities to retake operational control from Washington during wartime.

North Korea has condemned all exercises between Seoul and Washington as an invasion rehearsal and recently conducted four rounds of short-range ballistic missile tests as a "warning" to the allies.

"I got a very beautiful letter from Kim Jong-un yesterday," Trump said. "I think we'll have another meeting. He really wrote a beautiful, three-page -- I mean, right from top to bottom -- a really beautiful letter."

In the memo, Kim said he "wasn't happy with the war games, the war games on the other side with the United States," Trump said. "I've never liked it either ... I don't like paying for it. We should be reimbursed for it."

Trump and Kim have held three meetings to negotiate the dismantlement of North Korea's nuclear weapons program in exchange for sanctions relief.

At their last meeting at the inter-Korean border in June, the two agreed to resume working-level negotiations that stalled after their no-deal summit in Vietnam in February.

The talks have yet to be scheduled, but Trump has demonstrated a commitment to continuing diplomacy with the North.

"I say it again: There have been no nuclear tests. The missile tests have all been short-range. No ballistic missile tests. No long-range missiles," he said.

Since the North began firing short-range ballistic missiles last month, Trump has dismissed them as "very standard" despite UN Security Council resolutions banning the regime from ballistic missile launches of all kinds.

The president has insisted that Kim has kept his promise to refrain from nuclear and long-range missile tests, and held up their absence as one of his biggest diplomatic achievements ahead of the 2020 presidential election. 

"It was hand-delivered and it wasn't touched by anybody," Trump said of the letter. "They literally take it from North Korea to my office. We have a system. It's the old-fashioned system. You don't have to worry about leaks. Something nice about that system."

Trump also commented on the deteriorating ties between South Korea and Japan, saying he is "concerned" the two nations aren't getting along with each other.

"South Korea and Japan are fighting all the time. They've got to get along because it puts us in a very bad position," he said.

Washington has urged its two allies to resolve their trade row to prevent damage to trilateral security cooperation against North Korea's nuclear threats. 

Last month Trump said he had been asked by South Korean President Moon Jae-in to get involved and that he would be there if both Moon and Japanese Prime Minister Shinzo Abe needed him.

Seoul and Tokyo have seen their relations deteriorate to their worst condition in decades since Japan adopted export curbs against South Korea in early July.

The move came in apparent retaliation for South Korean court rulings ordering Japanese firms to compensate Korean victims of wartime forced labor.

This month Japan also removed South Korea from a list of trusted trading partners, sparking an angry response from Seoul, which threatened to end a military information-sharing agreement with Tokyo. (Yonhap)