SEOUL, South Korea -- North Korea on Friday announced it had built a "tactical nuclear attack submarine" as part of its effort to strengthen its naval force, the state news agency KCNA reported.

North Korean leader Kim Jong Un presided over the unveiling ceremony on Wednesday, saying the new sub was part of a "push forward with the nuclear weaponization of the Navy in the future", according to KCNA.

The launching 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", the KCNA report said, referring to the country by the abbreviation of its 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", KCNA reported.

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.

During the festive launching ceremony that involved confetti and balloons, Kim stressed "the strategic and tactical plan to continuously enhance the modernity of underwater and surface forces and push forward with the nuclear weaponization of the Navy in the future", KCNA said.

On Thursday, Kim inspected the submarine as it was preparing for a test cruise.

"Saying that to arm the navy with nuclear weapons arises as an urgent task... he stressed the need to enable the Navy to successfully carry out its strategic duty by hastening the transfer of underwater and surface vessels equipped with tactical nuclear weapons to the Navy", Kim was reported as saying by KCNA.

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 submarine fleets. However, experts doubt all of them are operational given the age of the vessels, according to NTI.