The launches, part of a record-breaking blitz of weapons tests this year by North Korea, came after Seoul's spy agency warned that Pyongyang was close to conducting another nuclear test.

South Korea's military said it had "detected two short-range ballistic missiles fired from the Sunan area in Pyongyang".

"Our military has reinforced monitoring and surveillance and is maintaining utmost readiness in close coordination with the United States," Seoul's Joint Chiefs of Staff said in a statement.

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

The latest launch comes after North Korea test-fired a short-range ballistic missile on Sunday.

Harris is due to arrive in Seoul on Thursday for a brief visit, during which she will go to the heavily fortified border between North and South Korea.

The White House has said Harris' visit intends to underscore the importance of the alliance with Seoul.

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

- 'Harbinger' -

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

Earlier Wednesday, Seoul's spy agency said Pyongyang appeared to have completed "a third tunnel at its Punggye-ri nuclear site", MP Yoo Sang-bum told reporters after a briefing from Seoul's National Intelligence Service.

Pyongyang is likely to choose the window between "China's upcoming Communist Party Congress on October 16 and the midterm elections in the United States on November 7" for its next nuclear test, Yoo said.

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.

Seoul has also detected signs the North is preparing to fire a submarine-launched ballistic missile (SLBM), the president's office said Saturday, a weapon Pyongyang last tested in May.

"Today's launch makes it clear the North is attempting to gain an upper hand on the peninsula with a nuclear arsenal at its disposal," Kim Jong-dae of the Yonsei Institute for North Korean Studies told AFP.

The repeated tests are "a harbinger of Pyongyang's aggressive posturing to come next month -- with missile launches and a possible nuclear test", he added.

Last week the nuclear-powered USS Ronald Reagan conducted joint drills with South Korea's navy in waters off the Korean peninsula.

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