North Korea has launched what it called its first operational "tactical nuclear attack submarine" and assigned it to the fleet that patrols the waters between the Korean peninsula and Japan, state media there said on Friday.

Submarine No 841 – named Hero Kim Kun Ok after a North Korean historical figure – will be one of the main "underwater offensive means of the naval force" of North Korea, leader Kim Jong Un said at the launch ceremony on Wednesday.

Analysts said the vessel appears to be a modified Soviet-era Romeo-class submarine, which North Korea acquired from China in the 1970s and began producing domestically. Its design, with 10 launch tube hatches, showed it was most likely armed with ballistic missiles and cruise missiles, analysts said.

At the launch ceremony, Kim said arming the navy with nuclear weapons was an urgent task and promised more underwater and surface vessels equipped with tactical nuclear weapons for the naval forces, news agency KCNA reported.

"The submarine-launching ceremony heralded the beginning of a new chapter for bolstering up the naval force of the DPRK," KCNA said, using the initials of the North's official name, the Democratic People's Republic of Korea.

North Korea plans to turn other existing submarines into nuclear armed vessels, and accelerate its push to eventually build nuclear-powered submarines, Kim said.

"Achieving a rapid development of our naval forces ... is a priority that cannot be delayed given ... the enemies' recent aggressive moves and military acts," the North Korean leader said in a speech, apparently referring to the United States and South Korea.

North Korea's nuclear weapons and ballistic missile programs are banned by United Nations Security Council resolutions, and the submarine launch drew condemnation from South Korea and Japan.

"North Korea's military activity is posing graver and more imminent threat to our country's security than before," Japanese Chief Cabinet Secretary Hirokazu Matsuno told a briefing. (Reuters)