North Korea has sent troops to Russia to join the fight against Ukraine, a major shift in Moscow’s effort to win the war, U.S. officials confirmed on Wednesday. Defense Secretary Lloyd J. Austin III called the North’s presence a “very, very serious” escalation that would have ramifications in both Europe and Asia.

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

       Mr. Austin cast President Vladimir V. Putin’s need for North Korean mercenaries as a sign of desperation.

       “This is an indication that he may be in even more trouble than most people realize,” he said. “He went tin-cupping early on to get additional weapons and materials from the D.P.R.K.,” he added, using the abbreviation for Democratic People’s Republic of Korea, “and then from Iran, and now he’s making a move to get more people.”

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       But he said intelligence analysts were still trying to discern whether the troops were moving toward Ukraine.

       Ukrainian officials insist they are headed there, and Ukraine’s defense minister was quoted on Wednesday saying he expected to see North Korean troops in Kursk, the Russian territory that Ukraine has occupied, in the coming days.

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