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 outbreak.

The string of weapons drills come as a prolonged hiatus in disarmament talks with the United States drags on, and despite recent overtures from Washington offering help to contain the pandemic.

South Korea's Joint Chiefs of Staff said the North fired the two projectiles, presumed to be ballistic missiles, from the port city of Wonsan into the Sea of Japan, also known as the East Sea.

"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 in a statement.

Tokyo's defence ministry said the projectiles fell short of Japanese waters and the country's exclusive economic zone.

The nuclear-armed North has yet to issue a statement on Sunday's missiles but attributed its three other launches this month - all of which landed in the Sea of Japan - to "long-range artillery" drills.

It last week fired what Seoul said were two short-range ballistic missiles, describing them as a new "tactical guided weapon".

North Korean state media announced the following day that the country's leader Kim Jong Un had received a letter from US President Donald Trump detailing a plan to develop ties - a move later confirmed by a White House official.

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.

Trump "explained his plan to propel the relations between the two countries... and expressed his intent to render cooperation in the anti-epidemic work," she said in a statement published by the Korean Central News Agency (KCNA).

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