South Korea had earlier detected signs the North was preparing to fire a Submarine Launched Ballistic Missile (SLBM), the president's office said Saturday, a weapon Pyongyang last tested in May.

The Sunday launch is the latest in a record-breaking blitz of weapons tests by nuclear-armed Pyongyang this year, including firing an Intercontinental Ballistic Missile at full range.

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

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

Japan's coast guard also confirmed a likely ballistic missile launch, citing information from Tokyo's defence ministry.

"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.

- Kamala Harris visit -

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.

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.

Yoon is also due to meet US Vice President Kamala Harris Thursday when she visits Seoul this week, following a visit by President Joe Biden in May, and US House Speaker Nancy Pelosi last month.

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.

"Pyongyang could be making a show of strength while a US aircraft carrier is visiting South Korea for defense exercises," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"But North Korea's major tests are, most of all, part of a long-term campaign for advancing offensive military capabilities."

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

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.

"North Korea might be delaying its seventh nuclear test out of respect for China's upcoming political conference that Xi Jinping is tightly scripting to extend his leadership," Easley said.

"But there are limits to Pyongyang's self-restraint."