North Korean leader Kim Jong-un has assured Russian President Vladimir Putin that their alliance will “invariably” continue, underscoring Pyongyang’s growing support for Moscow amid its war against Ukraine.

In a congratulatory letter marking Putin’s birthday, Kim said the “relations of alliance between the two countries, which have greeted the great heyday, will be invariably carried forward in the future.” He added that their partnership will contribute to establishing a “just and multi-polarized world order,” according to the Korean Central News Agency (KCNA).

Kim reaffirmed his “full support” for Russia’s “just struggle” to defend its sovereignty — a clear nod to Moscow’s invasion of Ukraine. South Korea’s intelligence agency has reported that North Korea has sent around 15,000 troops and weapons to assist Russia since October last year.

The letter follows earlier communications between the two leaders. In August, Kim and Putin held rare phone talks in which Kim pledged unwavering adherence to the DPRK-Russia treaty and continued support for all measures taken by Russia’s leadership.

The renewed rhetoric reflects deepening military and strategic cooperation between Pyongyang and Moscow, raising concerns in the West about arms transfers and the strengthening of anti-Western alliances.