North Korea has fired what appeared to be two short-range ballistic missiles, South Korea's military said, weeks after North Korea threatened to demonstrate a "new strategic weapon" and its deadline for the United States to offer sanctions relief expired.

The launch was the nuclear-armed North's first for more than three months and came as nuclear negotiations with the US remain at a standstill.

The two devices were fired eastwards over the sea from the Wonsan area on the east coast and flew 240 kilometres at a maximum altitude of 35km, the South's Joint Chiefs of Staff said.

They were "believed to be short-range ballistic missiles," a JCS official said.

South Korea's security ministers expressed "strong concern" the North was "carrying out actions giving rise to military tensions", the presidential Blue House said.

Japan's defence ministry said there was no indication of anything coming down in its waters or exclusive economic zone, but added: "Recent repeated launches of ballistic and other missiles by North Korea are a serious issue."

The launch came as North Korea battles to prevent a coronavirus outbreak and days after the one-year anniversary of the collapsed Hanoi summit between leader Kim Jong-un and US President Donald Trump.

Negotiations have since been deadlocked over sanctions relief and what the North would be willing to give up in return, despite a high-profile June meeting in the Demilitarized Zone that divides the peninsula.

North Korea carried out a series of weapons tests late last year, the last of them in November, often describing them as multiple launch rocket systems. It also carried out static engine tests, most recently in December.

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