North Korea fired at least one "unidentified projectile" into the sea off its east coast, the Yonhap news agency reported, citing the South's military Joint Chiefs of Staff.

The projectile was fired towards the Sea of Japan, also known as the East Sea, the report said, adding that additional details were not immediately available from military officials.

It was the fourth round of such launches this month by North Korea, as the world struggles with the coronavirus pandemic - and 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 missiles.

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

The report cited Mr Kim's sister Kim Yo Jong, who warned that the apparently good personal relationship between the two leaders would not be enough to kickstart 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 US administration official confirmed Mr Trump had sent a letter to Mr Kim, "consistent with his efforts to engage global leaders during the ongoing pandemic".

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

The deadlock is over sanctions relief and what North Korea would be willing to give up in return.

Pyongyang is under multiple sets of United Nations, US and other sanctions over its weapons programmes.