North Korea fired two short-range missiles into the sea on Thursday, complicating efforts to resume stalled nuclear talks with Washington and signalling its anger over planned US-South Korea joint military exercises.

It was the North's first missile test since an impromptu meeting between US President Donald Trump and North Korean leader Kim Jong Un last month that produced an agreement to resume a working-level denuclearisation dialogue.

But those talks have yet to begin, and Pyongyang warned recently they could be derailed by Washington and Seoul's refusal to scrap military exercises scheduled for next month.

South Korea's Joint Chiefs of Staff said the two missiles were launched just after dawn from Wonsan on the east coast and flew more than 430 kilometres (270 miles) before falling into the sea.

"Our military is closely monitoring the situation in case of additional launches and maintaining a readiness posture," it said.

"We urge the North to stop actions that do not help ease military tensions," said Choi Hyun-soo, a defence ministry spokeswoman for South Korea.

Japan's defence minister called the launches "extremely regrettable" but stressed that the missiles had fallen short of his country's exclusive economic zone.

Pyongyang carried out similar short-range launches in May, which Trump dismissed at the time as "very standard stuff" that would have no impact on his relationship with Kim.

The two leaders went on to hold an unscheduled meeting June 30 in the Demilitarized Zone that divides the two Koreas, where they agreed to pick up a nuclear dialogue that stalled after the collapse of a formal summit in Hanoi in February.

US Secretary of State Mike Pompeo said the working-level disarmament talks would probably start in mid-July, but last week Pyongyang said they had been jeopardised by the scheduled joint military drills.

Condemning the planned exercises as "blatant pressure" and a "violation of the spirit" of the joint statement Trump and Kim signed at their first summit in Singapore last year, Pyongyang even hinted it could reconsider its moratorium on nuclear and long-range missile testing.

Thursday's launches came a day after Trump's national security adviser, John Bolton, held talks with senior South Korean officials in Seoul.

Bolton's hawkish stance on dealing with Pyongyang has seen him regularly vilified by the North's state media.

The latest missile tests were a "strong message" that should be seen as "part of Pyongyang's protest" against the joint drills, said Cheong Seong-chang, an analyst at the Sejong Institute.

There are close to 30,000 US troops stationed in the South, and their annual manoeuvres with tens of thousands of South Korean soldiers have always infuriated the North which views them as provocative rehearsals for invasion.

Other analysts said opposition to the drills was merely a pretext for Pyongyang to pursue its weapons ambitions.

"I think today's launches are part of a larger plan for North Korea's advanced missile programme, rather than its protest against the upcoming military drill," said Hong Min, a senior researcher at the South's state-run Korea Institute for National Unification.

"The North has declared in the past it would modernise and improve its defence system.... the launches today are part of that development plan and I think the North had already planned the launches in advance."

North Korean state media published pictures this week of Kim inspecting a new submarine, fuelling concerns that Pyongyang was pushing ahead with the development of a submarine-launched ballistic missile (SLBM) programme.

Adam Mount, of the Federation of American Scientists, said the latest launch was a clear indication that "North Korea's nuclear and missile arsenals are now routinely being improved, displayed, and tested."

"The current bargain is: don't test nuclear warheads or long range missiles and the United States won't object or seriously try to stop it," Mount tweeted.

At their first summit in Singapore in June 2018 Trump and Kim adopted a vaguely worded statement on "complete de-nuclearisation of the Korean peninsula" and agreed to "establish new US-DPRK relations".

But the failure to reach an agreement over relief from US sanctions, and what the North was willing to give in return, led to the failure of their second summit in Hanoi.

Trump has repeatedly insisted that his personal relationship with Kim is strong enough to keep a dialogue with the North on track in the face of provocations like short-range missile launches.

Asked on Tuesday whether any meetings were scheduled, Trump said: "No, we just have a very good relationship. And probably they would like to meet and we'll see what happens."

"There was a little correspondence recently, very positive correspondence with North Korea." He said there had been "no nuclear testing, there's no missile testing, there's no nothing."