Pyongyang, North Korea- North Korean leader Kim Jong-un visited a nuclear submarine factory and received a message from Russia's Vladimir Putin hailing their "invincible friendship."

North Korea and Russia have drawn closer since Moscow launched its full-scale invasion of Ukraine nearly four years ago, and Pyongyang has sent troops to fight for Russia.

In return, Russia is sending financial aid, military technology and food, and energy supplies, according to analysts.

The "heroic" efforts of North Korean soldiers in Russia's Kursk region "clearly proved the invincible friendship" between Moscow and Pyongyang, Putin said in a message to Kim, according to the Korean Central News Agency (KCNA).

Their work demonstrated the two nations' "militant fraternity," the Russian leader said in the message received by Pyongyang last week.

The provisions of the "historic treaty" the two leaders signed last year, which includes a mutual defense clause, had been fulfilled "thanks to our joint efforts", Putin wrote.

KCNA reported Putin's letter on the same day that it published details of Kim's undated recent visit to a manufacturing base for nuclear-powered submarines.

There, the North Korean leader vowed to counter the "threat" of South Korea producing its own such vessels.

US President Donald Trump has given the green light for South Korea to build "nuclear-powered attack submarines," though key details of the project remain uncertain.

Photos published by KCNA showed Kim walking alongside a submarine described as weighing 8,700 tones at an indoor assembly site, surrounded by officials and his daughter Kim Ju Ae.

In another image, Kim Jong-un smiles during an official briefing as Kim Ju Ae stands beside him.

Pyongyang views Seoul developing nuclear subs as "an offensive act severely violating its security and maritime sovereignty", Kim Jong-un said, according to KCNA.

As a result, he insisted it was "indispensable" to "accelerate the radical development of the modernization and nuclear weaponization of the naval force."

Kim clarified a naval reorganization plan and learned about research into "new underwater secret weapons", KCNA said, without providing details.

Pyongyang's defense ministry said it would consider "countermeasures" against US "nuclear muscle flexing," a separate report said Thursday.

Kim also reportedly oversaw the test launch on Wednesday of "new-type high-altitude long-range anti-air missiles" over the East Sea, also known as the Sea of Japan.

The projectiles hit mock targets at an altitude of 124 miles, KCNA said. That height, if correct, would be in space.

One photo showed a missile ascending into the sky in a trail of intense orange flame, while another showed Kim walking in front of what appeared to be a military vehicle equipped with a vertical missile launcher.