North Korean leader Kim Jong Un has sent a birthday message to Russian President Vladimir Putin, calling him his “closest comrade” and expressing hopes for even stronger ties between their nations.

Kim’s message, in honor of Putin’s 72nd birthday, emphasized that their relationship has reached new heights, particularly since the Ukraine war, which has brought the two countries closer—raising concerns in the West.

In a separate statement on Tuesday, Kim also announced plans to accelerate North Korea’s efforts to become a military superpower with nuclear capabilities.

According to Yonhap News, citing North Korean state media KCNA, Kim praised the growing “invincible and eternal” alliance between North Korea and Russia, particularly since Putin’s visit to Pyongyang in June. Kim added that their meetings and strong bonds would continue to reinforce the foundation of their friendship.

The relationship between North Korea and Russia dates back to the Soviet era, with Moscow offering early support to North Korea in its development. Over the years, Pyongyang has often favored Russian assistance over that of China, with whom it maintains a more cautious relationship.

Earlier this year, Kim and Putin signed an agreement to support each other in the face of “aggression,” though specifics on what would qualify as such were not provided. There have also been allegations that North Korea has aided Russia in its war against Ukraine by supplying weapons in exchange for economic and technological help. Reports have emerged that Russia has been using North Korean missiles in Ukraine.

Jeffrey Lewis, director at the James Martin Center for Nonproliferation Studies, noted that both leaders are working to “reduce the pain of international sanctions” by fostering relationships with nations outside U.S. influence.

During Kim’s visit to Russia in September 2023, Putin pledged to assist North Korea with its satellite program, following multiple failed launches by Pyongyang.

In a related development, South Korean lawmaker Kang Dae-sik stated on Tuesday that intelligence suggests North Korea has begun building a potential nuclear-powered submarine, though construction remains in its early stages and requires further verification.