"North Korea fired an unidentified ballistic missile," Seoul's Joint Chiefs of Staff said early Sunday, without giving any further details.

The launch, part of a record-breaking blitz of weapons tests this year by the North, also comes after reports indicated that Pyongyang could be preparing to fire a submarine-launched ballistic missile.

Japan's coast guard also confirmed a possible ballistic missile launch, citing information from Tokyo's defence ministry, and warned ships to be aware.

"Vessels please be vigilant for new information and if you spot any foreign objects please don't get closer to them but inform the coast guard," the coast guard said.

Japan's public broadcaster NHK said the object appeared to have fallen outside Japan's exclusive economic zone.

On Friday, the nuclear-powered USS Ronald Reagan and vessels from its strike group docked in the southern port city of Busan, part of a push by Seoul and Washington to have more US strategic assets operating in the region.

South Korea's hawkish President Yoon Suk-yeol, who took office in May, has vowed to beef up joint military exercises with the United States, after years of failed diplomacy with North Korea under his predecessor.

The USS Reagan will take part in joint drills off South Korea's east coast this month.

Washington is Seoul's key security ally and stations about 28,500 troops in South Korea to protect it from the North.

The two countries have long carried out joint exercises, which they insist are purely defensive but North Korea sees them as rehearsals for an invasion.

South Korean and US officials have been warning for months that North Korean leader Kim Jong Un is preparing to conduct another nuclear test.

In May, the North test-fired a short range ballistic missile from Sinpo, a major naval shipyard in North Korea.

The isolated regime has tested nuclear weapons six times since 2006. Its last and most powerful one in 2017 -- which Pyongyang claimed was a hydrogen bomb -- had an estimated yield of 250 kilotons.