North Korean leader Kim Jong Un presided over the unveiling ceremony on Wednesday, saying the sub was part of a "push forward with the nuclear weaponisation of the Navy", according to state news agency KCNA.

Images in state media showed Kim, wearing a light suit and sun hat, speaking to white-uniformed sailors next to the submarine, whose bow was decorated with the North Korean flag.

The launch of submarine No. 841 -- named the Hero Kim Kun Ok -- "heralded the beginning of a new chapter for bolstering up the naval force of the DPRK", KCNA said, referring to the abbreviation of North Korea's formal name.

Kim said the sub "will perform its combat mission as one of core underwater offensive means of the naval force of the DPRK", the agency added.

North Korea will turn its existing submarines into attack vessels equipped with nuclear weapons, Kim said.

On Thursday, KCNA said Kim inspected the submarine as it prepared for a test cruise, and entered it "to acquaint himself with its weapon system and underwater operation capability".

During the launch ceremony, which involved confetti and balloons, Kim stressed "the strategic and tactical plan to continuously enhance the modernity of underwater and surface forces", KCNA said.

- Modified sub -

North Korea has conducted a record number of weapons tests this year, and last month failed in its second attempt to put a spy satellite into orbit.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets, and holding naval drills with Japan.

According to the Nuclear Threat Initiative, a US-based think tank, North Korea is estimated to have between 64 and 86 submarines, one of the world's largest fleets.

However, experts doubt if all of them are operational given their age, according to NTI.

In 2019, Kim was shown in state media inspecting a previously unreported submarine.

"This is the same -- albeit more extensively modified -- submarine North Korea showed us way back in July 2019," Joseph Dempsey, a researcher at the International Institute for Strategic Studies, wrote on X.

"While North Korea has added a missile compartment and externally emulated more contemporary design features... (the sub) at its core is an obsolete Romeo class diesel-electric boat, originally designed in the 1950s," he said.

"As a platform it will have some fundamental limitations and vulnerabilities."

The capacities of the new submarine "won't be revolutionary, but will increase the complexity of the nuclear threat posed by North Korea," said US-based analyst Ankit Panda.

"Kim Jong Un early in his tenure indicated that he viewed the Korean People's Navy's role as significant and he's intermittently focused on indicating that publicly."

South Korea condemned the submarine launch.

North Korea was "squandering its scarce resources into its futile weapons development while disregarding living difficulties of its people", said Kim In-ae, deputy spokesperson of South Korea's Unification Ministry.

"Pyongyang must realise its weapons programmes and threats... only puts their security at risk in the face of an overwhelming response from the strengthened South Korea-US-Japan joint posture."