Trump said he had just received a "beautiful letter" from Kim expressing Pyongyang's anger over US-South Korean joint war games, which spurred the series of four missile tests in two weeks.

While other US officials called the launches "provocations," Trump said he agreed with the North Korean leader and was hoping to meet him again, with talks having been frozen for months.

"I got a very beautiful letter from Kim Jong Un yesterday," Trump said. "It was a very positive letter."

"He wasn't happy with the war games," Trump said, referring to new military exercises between US forces and the South Korean military that began this week.

"As you know, I've never liked it either. I've never been a fan. And you know why? I don't like paying for it."

- Solemn warning -

It was a generous response to Kim, whose nuclear-armed government called the missile launches a "solemn warning" over the joint war games that began this week.

The drills were a "flagrant violation" of the diplomatic process between Pyongyang, Washington and Seoul, Kim's government said.

Trump's response also contrasted with his tough words for Washignton's key East Asia allies, who were locked in a searing trade row.

"South Korea and Japan are fighting all the time," he said. "They have to get along with each other. If they don't get along, what are we doing?"

- Talks faltered after Singapore -

Trump has appeared determined to secure a denuclearization agreement with North Korea ahead of the November 2020 US presidential elections despite faltering talks since he first met Kim in a historic ice-breaking summit in Singapore in June 2018.

At the time the US temporarily froze military exercises with South Korea and Trump claimed Kim had agreed to give up his newly-acquired nuclear weapons and long-range ballistic missiles.

But Pyongyang has maintained that the United States must lift its economic embargo and sanctions on the country to make any progress in nuclear talks.

Yet even after their abortive second summit in Hanoi in February, Trump has been loath to criticize the North Korean leader.

In June he offered an olive branch by meeting Kim in the Panmunjon truce village on the North Korea-South Korea border, the first sitting US president ever to step inside the North.

And on Friday, he said the missile launches weren't important.

"I'll 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," Trump said.

- Missile 'provocations' -

But the broader Trump administration's position is less enthusiastic.

After Pyongyang's fourth missile launch early this week, Secretary of State Mike Pompeo said Washington was ready to resume talks and that their strategy for the "full, final denuclearization of North Korea" had not changed.

A senior State Department official who refused to be named told journalists last week however that the missile tests were an impediment to peace.

"The missile launches, any kind of provocations, are not helpful to advancing the cause of diplomacy," the official said.

The official said there was a unified message going to the North Koreans from China and Russia that they need to "cease the provocations, reengage in diplomacy to achieve complete denuclearization."

Meanwhile, the adminsitration's lead North Korea negotiator, Stephen Biegun, could step down after just one year to take a new diplomatic job, media reports said this week.

Critics say Trump, in his desire for a deal, is allowing Kim too much ground.

"Kim is playing him masterfully," by launching missiles one day and then flattering Trump with a letter just days later, said Vipin Narang, a Massachusetts Institute of Tehcnology profressor who follows US-North Korea nuclear talks closely.

Kim "can run this play indefinitely because it always works."