Three devices were fired eastwards over the sea from the Sondok area in South Hamgyong province, the South's Joint Chiefs of Staff said in a statement.

"The military is monitoring for additional launches and maintaining readiness," it added.

Monday's launch was the North's second weapons test in a week.

After last week's launch the North's state media said leader Kim Jong Un had overseen a "long-range artillery" drill but it was not clear if those two incidents last week were the same.

The latest launch came days after Kim sent a personal letter to the South's President Moon Jae-in, offering "comfort" for the rapid outbreak of the new coronavirus outbreak in the country.

That message had followed an unprecedented statement by Kim's younger sister Yo Jong, when she said Seoul's condemnation of Pyongyang's weapons test last week was a "truly senseless act" and "perfectly foolish".

The North carried out a series of weapons tests late last year, the last of them in November, which it often described as multiple launch rocket systems although others called them ballistic missiles.

It also carried out static engine tests, most recently in December.

At a party meeting at the end of that month, Kim declared that Pyongyang no longer considered itself bound by its moratoriums on nuclear and intercontinental ballistic missile tests, and threatened a demonstration of a "new strategic weapon" soon.

The launches come as nuclear negotiations with the US are at a standstill and after the expiry of a unilateral deadline Pyongyang set for Washington to offer it fresh concessions on sanctions relief.

The one-year anniversary of the collapsed Hanoi summit between Kim and US President Donald Trump also passed, earlier this year.

Pyongyang is under multiple sets of sanctions over its weapons programmes from the United Nations Security Council, US, South Korea and others.

Heightened tensions in 2017 were followed by two years of nuclear diplomacy between Pyongyang and Washington, including three meetings between Kim and Trump, but little tangible progress was made.