Pyongyang has conducted a string of weapons tests, including hypersonic missiles, as leader Kim Jong-un pursues his avowed goal of further strengthening the military.
The United States has called on North Korea to "cease its unlawful and destabilising activities," after Pyongyang fired two suspected ballistic missiles in its fourth weapons test this month.
In a call with South Korean and Japanese officials on Monday, the US special representative on North Korea, Sung Kim "expressed concern" about the missile launches.
Kim urged Pyongyang to return to dialogue "without preconditions," a statement from spokesman Ned Price said.
He also "reaffirmed the US commitment to the complete denuclearisation of the Korean Peninsula, as well as its ironclad commitment to the defence of its allies," South Korea and Japan.
Two suspected "short-range ballistic missiles" were fired east from an airport in Pyongyang early on Monday, the South Korean military's Joint Chiefs of Staff said, with Japan also confirming the launch.
Fired just before 9 am local (0000 GMT), they flew 380 kilometres (around 240 miles) at an altitude of 42 km, the JCS added.
Despite biting international sanctions, Pyongyang has conducted a string of weapons tests this year, including hypersonic missiles, as leader Kim Jong-un pursues his avowed goal of further strengthening the military.
The frequent and varied tests this year indicate North Korea "is trying to improve its technology and operational capability in terms of covert actions", Japanese Defence Minister Nobuo Kishi told reporters.
Pyongyang said it successfully tested hypersonic gliding missiles on January 5 and January 11, with the second launch personally supervised by Kim.
Reeling economically from a self-imposed coronavirus blockade, impoverished North Korea has not responded to Washington's offers of talks.
Fresh US sanctions
The United States last week imposed fresh sanctions on five North Koreans connected to the country's ballistic missile programmes, prompting an angry reaction from Pyongyang.
A North Korean Foreign Ministry spokesman described the move as a "provocation", according to state news agency KCNA.
If "the US adopts such a confrontational stance, the DPRK will be forced to take stronger and certain reaction to it," the spokesman said hours before Pyongyang fired off two train-launched missiles on Friday.
Analysts said the Monday test also appeared to be an attempt to send the United States a message.