North Korean leader Kim Jong Un has sent a birthday message to Russian President Vladimir Putin, calling him his "closest comrade".
Kim, congratulating Putin on his 72nd birthday, added that relations between both countries would be raised to a new level.
Relations between Pyongyang and Moscow have deepened since the start of the Ukraine war - in a move that has worried the West.
Separately on Tuesday, Kim said Pyongyang would speed up steps to make his country a military super power with nuclear weapons.
According to Yonhap News quoting North Korean state media KCNA, Kim praised relations between both countries, saying they had become "invincible and eternal", since Putin's visit to Pyongyang in June.
"Meetings and comradely ties between us... will make a positive contribution to further consolidating the eternal foundation of the DPRK-Russia friendship," he added, referring to North Korea by its official name, the Democratic People's Republic of Korea.
The relationship between North Korea and Russia goes back decades - to Stalin and Kim Il-sung, the current Kim's grandfather. The Soviet Union supported North Korea in its early days with weapons and technology, and Pyongyang has never wanted to completely rely on China - which it does not fully trust.
Earlier this year, Putin and Kim signed an agreement pledging that they would help each other in the event of "aggression" against either country - though it was unclear what would constitute aggression.
Kim has been accused of helping Russia in the war against Ukraine by supplying it with weapons in exchange for economic and technological assistance.
There has been growing evidence that Russia has been deploying North Korean missiles in Ukraine.
For Putin, the relationship is likely more tactical than strategic. He needs support for his war in Ukraine and North Korea will certainly be willing to sell him whatever munitions he is willing to pay for.
Jeffrey Lewis, a director at the James Martin Center for Nonproliferation Studies, had earlier said both Kim and Putin were "trying to reduce the pain of international sanctions by creating an alternate network of friends and partners beyond the reach of US sanctions".
North Korea certainly can benefit enormously from access to Russian military technology – which despite Russia’s problems is still far more advanced than North Korea’s homegrown and reverse-engineered systems.
North Korea is primarily trying to perfect its nuclear and missile arsenal. It has made very surprising progress in the last decade in producing new missile systems and miniaturising its nuclear devices.
But Russia has far more advanced thermonuclear warhead designs, re-entry vehicle designs and solid rocket motor designs.
During a visit by Kim to Russia in September 2023, Putin had promised to help North Korea develop its satellites, after several failed launches by Pyongyang.
A South Korean lawmaker also separately on Tuesday claimed that Seoul's military had detected apparent signs that North Korea had started to build a possible nuclear-powered submarine.
Citing Korea's intelligence agency, Rep Kang Dae-sik said construction was still in its early stages and "further confirmation is needed on whether it is nuclear powered".
