North Korea's new leader Kim Jong-Un has visited the naval unit that captured a US spy ship in 1968, the official news agency said Friday, amid tensions over its planned satellite launch.

Any attempt to intercept the satellite would be "an act of war", the North said late Thursday after South Korea and Japan prepared to shoot down the rocket should it fall towards their territory.

In the latest of a series of military visits, Kim inspected Navy Unit 155 in the southeastern province of Kangwon and highly praised its past feats, the agency said.

It "startled the world" by sinking the US heavy cruiser Baltimore with just four torpedo craft during the 1950-53 Korean War and also captured the US spy ship Pueblo, it said.

Kim "stressed the need for the seamen of the unit to firmly take over the baton of the revolutionary forerunners (and) send the enemies into the bottom of the sea if they dare intrude into the territorial waters".

The North captured the spy ship with 83 crew members after it allegedly intruded into its territorial waters, sparking a Cold War crisis.

The crew were detained for 11 months before being released but the ship is still held by the North and moored on the Taedong River in Pyongyang.

Kim, supreme commander of the North's 1.2-million-strong military, earlier inspected a unit guarding Ryo Island near the southeastern port of Wonsan.

He urged troops there to "protect the island with barricades of iron and bury the enemy in the water if they invade", the news agency said in a report Wednesday.

Analysts said such trips and the upcoming launch are aimed at bolstering military loyalty to the young and untested leader.

The North says it will launch a peaceful satellite between April 12-16 to mark the 100th anniversary of the birth of its late founder Kim Il-Sung.

The United States and other countries see it as a pretext for a long-range missile test banned under US resolutions.

An official North Korean body in charge of cross-border relations, The Committee for the Peaceful Reunification of Korea, warned Thursday that interception of the satellite would be "an act of war" and would cause a "tremendous catastrophe".

Seoul and Tokyo say they will open fire on the rocket only if it threatens to fall on their territory.