North Korean leader Kim Jong Un, in a New Year's message to Russian President Vladimir Putin, said that the two nations have joined into "the sincerest alliance of sharing blood, life, and death in the same trench" while fighting together against Ukraine.

In the letter, carried by North Korea's state-run Korean Central News Agency, Kim also said that the two nations' "absolute solidity and mightiness" had been "more vividly etched in the pages of the times and history," and called their bond "invincible," the New York Post reported on Saturday.

In recent months, North Korea has sent thousands of troops to support Russia's war effort, including in the Kursk region, where Moscow has worked to push back Ukrainian forces.

Putin has also praised the North Korean alliance, sending Kim a New Year's letter in which he commended North Korea's "heroic" soldiers and lauded the two countries' "invincible friendship," according to KCNA.

Russia has faced broad international isolation and pressure from U.S. and European sanctions since the invasion began.

North Korea, already heavily sanctioned over its nuclear and missile programs, has deepened ties with Moscow as the war drags on.

CNN has reported that more than 6,000 North Korean troops have died so far in the fighting.