The two devices were fired eastwards over the sea from the Wonsan area on the east coast, the South's Joint Chiefs of Staff said.

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

The North carried out a series of weapons tests late last year, the last of them in November, often describing them 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, leader Kim Jong Un 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 move came with nuclear negotiations with the US at a standstill and as a unilateral deadline Pyongyang set Washington to offer it fresh concessions on sanctions relief expired.

Pyongyang has previously fired missiles capable of reaching the entire US mainland and has carried out six nuclear tests, the last of them 16 times more powerful than the Hiroshima blast, according to the highest estimates.

It is under multiple sets of sanctions over its weapons programmes from the United Nations Security Council, US, South and other bodies.

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