North Korea has conducted the latest in a series of missile launches to protest US-South Korean war games, just hours after US President Donald Trump expressed his own frustration with the exercises.

Defence officials in Seoul said what appeared to be two short-range ballistic missiles were fired at daybreak from near the northeastern city of Hamhung, flying 400 kilometres before splashing down in the sea between the Korean peninsula and Japan.

It was the fifth round of launches in two weeks, with North Korean leader Kim Jong-un labelling them a "solemn warning" over the joint military drills that began just days ago.

Today's tests came shortly after President Trump said he agreed with Kim's opposition to the war games - albeit for financial rather than military reasons.

Mr Trump has repeatedly talked up his close personal relationship with Kim, as his administration seeks to resume stalled denuclearisation talks with the North.

New Pentagon chief Mark Esper (L) visits South Korea while N.Korea protests military exercises

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

"He wasn't happy with the war games. I've never liked it either. I've never been a fan. And you know why? I don't like paying for it."

Hours later, the White House had no immediate comment on the newest rocket tests.

"We are consulting closely with our Japanese and South Korean allies," a senior administration official said.

The joint drills and ongoing military exercises in North Korea left a "high" possibility of further missile launches, Seoul's military joint chiefs of staff said in a statement.

"Our military is monitoring the situation in case of additional launches while maintaining a readiness posture," the statement said.

Donald Trump has appeared determined to secure a denuclearisation agreement with North Korea ahead of next year's presidential elections, despite faltering talks since he first met Kim in a historic ice-breaking summit in Singapore in June 2018.

Trump claimed Kim had agreed to give up his arsenal of 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 for talks to progress.

Yet even after their abortive second summit in February, Mr Trump has been reluctant to criticise the North Korean leader.

In June he offered an olive branch by meeting Kim in the Panmunjon truce village in the Korean Demilitarised Zone, becoming the first sitting US president ever to step inside the North.

On Friday, he said the missile launches were not 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," Mr Trump said.

Washington and Seoul pledged in March to scale down its joint drills in an effort to foster denuclearisation talks with Pyongyang.

Read more:
North Korea fires two ballistic missiles
North Korea 'fires two unidentified projectiles'

Leif-Eric Easley, a professor at Ewha Woman's University in Seoul, said that Kim had used personal appeals to President Trump blaming the tests on the joint military exercises hoping to drive a wedge between Washington and South Korea.

"Breaking the alliance is exactly what Pyongyang wants, which is why it makes all this noise," Mr Easley said.

Others in the Trump administration have voiced their opposition to the launches.

After Pyongyang's fourth launch early this week, Secretary of State Mike Pompeo said Washington was ready to resume talks, but that the target of the "full, final denuclearisation of North Korea" had not changed.

A senior State Department official, who refused to be named, told journalists last week 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.