As talks with the United States remain stalled, Pyongyang looks set on military modernisation, testing hypersonic missiles twice this month and firing train-borne missiles Friday in response to new sanctions.

South Korea's military detected two suspected "short-range ballistic missiles fired eastward from the Sunan airport in Pyongyang", Seoul's Joint Chiefs of Staff said Monday.

Japan's coastguard also detected the launch of "a possible ballistic missile", a spokesman told AFP.

Since North Korea's leader Kim Jong Un vowed to continue building up the country's defence capabilities at a key meeting of the ruling party last month, Pyongyang has embarked on a string of new tests.

It said it had successfully tested a hypersonic gliding missile on January 5 and January 11.

Kim, who personally oversaw the second launch, then called for the development of more "strategic military muscle" after the test.

In response, the United States last week imposed fresh sanctions on five North Koreans connected to the country's ballistic missile programmes.

This prompted an angry reaction from Pyongyang, with a foreign ministry spokesman saying the sanctions were a "provocation" and accusing the US of "intentionally escalating" the situation.

If "the US adopts such a confrontational stance, the DPRK will be forced to take stronger and certain reaction to it," the spokesman said in comments carried by state news agency KCNA early Friday in North Korea.

The latest test appeared to be another attempt to send the United States a message that Pyongyang will not tolerate any infringement on its right to self-defence, analysts told AFP.

"The North appears to be sending the US a message in response to the sanctions... it is signalling that it will forge ahead with tests despite criticism," said Hong Min of the Korea Institute for National Unification in Seoul.

- Train to China? -

Despite biting international sanctions over its weapons programmes, Pyongyang has refused to respond to US appeals for talks.

Dialogue between Washington and Pyongyang remains stalled, and impoverished North Korea is also under a self-imposed coronavirus blockade that has hammered its economy.

The new test comes after a North Korean freight train crossed the Yalu River railroad bridge into China Sunday, according to the Yonhap news agency.

The move could signal the prospect of resumed China-North Korea land trade, which has been suspended for more than a year due to the global pandemic.

"Amidst a flurry of missile launches, North Korea appears to have resumed cross-border trade with China via train," said Leif-Eric Easley, a professor at Ewha Womans University in Seoul.

"This timing suggests Beijing is more than complicit with Pyongyang's provocations; China is supporting North Korea economically and coordinating with it militarily."