North Korea's leader Kim Jong Un said his country will always support Russia, and complimented his counterpart in a letter published by state media on Wednesday.
Ties between the two countries have grown closer since Moscow's 2022 invasion of Ukraine, with Pyongyang dispatching ground troops and weapons systems to aid Russia's war effort.
Analysts say the assistance has been provided in exchange for Russia's provision of food and weapons technology.
"I express my sincere thanks to you for sending warm and sincere congratulations first on my reassumption of the heavy duty as President of the State Affairs," Kim said in the message on Tuesday, the official Korean Central News Agency said.
North Korea's rubber-stamp legislature reappointed Kim this week as head of the authoritarian nation's highest policymaking and governing body.
"Today, the DPRK and Russia are closely cooperating to defend the sovereignty of the two countries," Kim said, using the initials of the North's official name.
"Pyongyang will always be with Moscow. This is our choice and unshakable will," he added.
South Korean and Western intelligence agencies have estimated that the North has sent thousands of soldiers to Russia, primarily to the Kursk region, along with artillery shells, missiles and long-range rocket systems.
Around 2,000 troops have been killed and thousands more have been wounded, according to South Korean estimates.
Putin and Kim struck a military deal during Putin's visit to Pyongyang in June 2024, obligating both states to provide military assistance "without delay" in the event of an attack on the other.
Kim's letter follows Belarusian state media reports that President Alexander Lukashenko would visit North Korea on a two-day trip from Wednesday "to strengthen bilateral cooperation".
Both countries have provided support for Russia in its war on Ukraine and are adversaries of the West.
