North Korean leader Kim Jong Un visited a nuclear submarine factory and received a message from Russia's Vladimir Putin hailing the countries' "invincible friendship," Pyongyang's state media said Thursday.
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 North Korea financial aid, military technology and food and energy supplies, analysts say.
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 nations' "militant fraternity", Putin 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.
Photos published by KCNA showed Kim walking alongside a purportedly 8,700-ton submarine at an indoor assembly site, surrounded by officials and his daughter Kim Ju Ae.
