SEOUL/TOKYO: North Korea fired a ballistic missile on Saturday, South Korea and Japan said, three days before the inauguration of South Korean president-elect Yoon Suk-yeol, who has vowed to take a hard line against the North.

South Korean military said that North Korea fired what is believed to be an submarine-launched ballistic missile (SLBM) into the sea off its east coast around 0507 GMT on Saturday from a submarine around Sinpo, where North Korea keeps submarines as well as equipment for test-firing SLBMs.

Japan's Defence Minister Nobuo Kishi said North Korea's recent development in nuclear missile-related technology and repeated launches of ballistic missiles threatened the region and the international community.

"This is absolutely unacceptable," he told reporters, adding that Japan will continue to "strengthen defence capabilities drastically" to protect citizens from such security threats.

N Korean leader Kim observes missile test to enhance nuclear capabilities

Kishi said it is possible for North Korea to complete nuclear test preparations as early as this month, and take further provocative acts.

This was in line with a U.S. assessment that Pyongyang was preparing its Punggye-ri nuclear test site and could be ready to conduct a test there as early as this month.

Yoon takes office on Tuesday. U.S. President Joe Biden is to visit South Korea and meet with him on May 21, a day before travelling to Japan where he meets Japanese Prime Minister Fumio Kishida.

"This is aiming at the (South's) new administration beginning next week, and applying preemptive pressure to take control of the situation before the U.S.-South Korea summit," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

"It also creates tension to strengthen the regime's internal coherence in the face of circumstances such as prevention of COVID-19 spreading."

North Korea signals expansive mission for its nuclear weapons

Japan and South Korea estimated Saturday's SLBM had flown as high as 50-60 km (30-40 miles) and as far as 600 km (370 miles).

On Wednesday, North Korea fired a ballistic missile toward the sea off its east coast, South Korea and Japan said, after Pyongyang vowed to develop its nuclear forces "at the fastest possible speed".

"Instead of accepting invitations to dialogue, the Kim regime appears to be preparing a tactical nuclear warhead test. The timing will depend most on when the underground tunnels and modified device technology are ready," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"A seventh nuclear test would be the first since September 2017 and raise tensions on the Korean Peninsula, increasing dangers of miscalculation and miscommunication between the Kim regime and the incoming Yoon administration."

Last month, North Korean leader Kim Jong Un pledged to speed up development of his country's nuclear arsenal. He presided over a huge military parade that displayed intercontinental ballistic missiles as well as what appeared to be SLBMs being carried on trucks and launch vehicles.

In October, North Korea test-fired a new, smaller ballistic missile from a submarine, a move that analysts said could be aimed at more quickly fielding an operational missile submarine.

North Korea fires new ICBM in largest test since 2017

Yoon, in an interview with Voice of America released on Saturday, said that a meeting with Kim Jong Un is not off the table but would need to have concrete results.

"There's no reason to avoid meeting" Kim, Yoon said. "However, if we are not be able to show any results, or results are just for show and does not have actual results in denuclearisation... it's not going to help the relationship between the two Koreas progress."