North Korea fired what appeared to be two short-range ballistic missiles off its east coast on Sunday, the fourth such launch this month as the world battles the coronavirus pandemic.

Two projectiles were fired eastwards from the port city of Wonsan and flew 230 kilometers into the Sea of Japan — also known as the East Sea — at a maximum altitude of 30 kilometers, South Korea's Joint Chiefs of Staff said in a statement.

"Such military action by North Korea is an extremely inappropriate act when the entire world is having difficulties due to the COVID-19 outbreak," they added.

Japan's Defense Ministry said the "ballistic missile-like objects" did not cross into Japanese waters or the country's exclusive economic zone.

The latest launch by Pyongyang comes as a prolonged hiatus in disarmament talks with the United States drags on.

A little over a week ago, the nuclear-armed North fired what were believed to be two short-range ballistic missile, describing them as a new "tactical guided weapon."

A day later, North Korean state media announced that U.S. President Donald Trump had sent a letter to North Korean leader Kim Jong Un detailing a plan to develop ties.

The report cited Kim's powerful sister Kim Yo Jong, who warned that the apparently good personal relationship between the two leaders would not be enough to foster broader relations.

"In the letter, he… explained his plan to propel the relations between the two countries of the DPRK and the US and expressed his intent to render cooperation in the anti-epidemic work," an apparent reference to the coronavirus pandemic, she said in the statement reported by the North's Korean Central News Agency (KCNA).

A senior U.S. administration official confirmed Trump had sent a letter to Kim, "consistent with his efforts to engage global leaders during the ongoing pandemic".

North Korea has one of the few remaining countries in the world yet to report a case of novel coronavirus infection.

But the outbreak has turned into a major international crisis, with more than 640,000 confirmed cases and 30,000 dead worldwide.

Analysts say the North has been continuing to refine its weapons capabilities more than a year after a summit between Kim and Trump broke down in Hanoi.

Negotiations have since been deadlocked over sanctions relief and what the North would be willing to give up in return.

Pyongyang set a unilateral end-2019 deadline for Washington to offer fresh concessions, and in late December, Kim declared his country no longer considered itself bound by moratoriums on nuclear and intercontinental ballistic missile tests.

North Korea is under multiple sets of United Nations, U.S. and other sanctions over its weapons programs.

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