North Korea fired two short-range missiles into the sea, South Korea says

North Korea has fired two short-range missiles from its east coast, a South Korean official says, its first missile tests since leader Kim Jong-un and US President Donald Trump agreed to revive stalled denuclearisation talks.

North Korea launched the missiles on Thursday from the coastal city of Wonsan with one flying about 430km and the other 690km out over the sea.

They both reached an altitude of 50km before splashing down, an official at South Korea's defence ministry told Reuters.

The second, longer-range missile appeared to be a new design, but a detailed analysis was being done to verify that, the official said.

The firing of the missiles will cast new doubt on efforts to restart denuclearisation talks after Trump and Kim met at the demilitarised zone (DMZ) between the two Koreas at the end of June.

The White House, Pentagon and US State Department did not immediately respond to requests for comment.

A US official, who declined to be identified, said according to initial information at least one short-range projectile was fired from North Korea. Further analysis was underway, the official said.

South Korea had detected related signs prior to the launch and was conducting detailed analysis with the United States, the presidential Blue House said in a statement.

South Korea's defence ministry urged the North to stop acts that are unhelpful for easing tension, saying the latest test posed a military threat.

Trump's national security adviser John Bolton, who has taken a hardline towards North Korea, made no mention of the launches in a tweet on Thursday after a visit to South Korea.

He said he had "productive meetings" with Seoul officials on regional security and building a stronger alliance.

South Korea's nuclear envoy, Lee Do-hoon, had phone calls with his US counterpart, Stephen Biegun, and his Japanese counterpart, Kenji Kanasugi, to share their assessment of the situation, South Korea's foreign ministry said in a statement.

North Korea's last weapons testing in May included short-range missiles as well as smaller rockets.

At the time, Kim oversaw the first flight of a previously untested weapon - a relatively small, fast missile that experts believe will be easier to hide, launch and manoeuvre in flight.