Five months ago, the autocratic leaders of Russia and North Korea signed a treaty on mutual defense and cooperation, deepening ties between the two countries that stretch back beyond the Cold War.

       Now, North Korea’s leader, Kim Jong-un, has deployed soldiers to assist Moscow’s war effort in Ukraine. More than 600,000 Russian troops have been killed and wounded since President Vladimir V. Putin launched the full-scale invasion of Ukraine in February 2022.

       On Wednesday, the United States confirmed that North Korean troops had landed in Russia to join the fight against Ukraine, a major shift in Moscow’s effort to win the war. Secretary of Defense Lloyd J. Austin III called the North’s presence a “very serious” escalation that would have ramifications in both Europe and Asia.

       “What exactly are they doing?” Mr. Austin told reporters at a military base in Italy. “Left to be seen.” He gave no details about the number of troops already there or the number expected to arrive.

       Advertisement

       SKIP ADVERTISEMENT

       North Korean soldiers started reaching the Russian Far East this month, sailing on Russian Navy ships, according to South Korea’s intelligence agency. There are 3,000 North Korean soldiers on Russian soil at the moment, the agency said on Wednesday, and their numbers are expected to swell to 10,000 by December.

       This week President Volodymyr Zelensky of Ukraine suggested that North Korea was preparing 12,000 soldiers to fight on the Russian side.

       Subscribe to The Times to read as many articles as you like.