SEOUL, South Korea — New reports from Kyiv say that North Korea is deploying troops to Russia that could be used to fight in Ukraine, with some 10,000 North Korean troops now undergoing training in Russia’s Far East.

The account is the latest in a string of recent signs of tightening security ties between two prime U.S. adversaries. Last week, there were unconfirmed reports that six North Korean officers had been killed by a Ukrainian strike missile in the raging war with Russia across Ukraine’s occupied southern and eastern regions.

A South Korean military official told the Yonhap News Agency that the new reports were being “closely monitored.”

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Both South Korea and the United States have already accused the regime of North Korean leader Kim Jong-un of supplying massive quantities of artillery and rocket artillery munitions to Moscow for the Ukraine conflict.

A strategic partnership signed by North Korea and Russia in Pyongyang in June calls for each party to aid the other if invaded. The Ukrainian surprise military incursion into Russia’s Kursk region may trigger the treaty. On Monday, Russian President Vladimir Putin presented the treaty to the State Duma for ratification.

A fog of uncertainty still clings to accounts that North Korean troops are set to take part in the fighting alongside their Russian allies.

The Kyiv Post, quoting Ukrainian intelligence sources, on Wednesday reported that some 10,000 North Korean troops were already receiving training in Russia’s Republic of Buryatia — more than triple the estimate provided a day earlier. Their training facility is the base of Russia’s 11th Independent Airborne Brigade, and the North Korean troops are being formed into a specialist “Buryat Battalion,” according to the newspaper.

U.S. Deputy Secretary Kurt Campbell, in Seoul Wednesday for talks with regional allies on North Korea, told reporters the Biden administration was “alarmed” by Pyongyang’s recent nuclear and missile threats and its growing ties to Russia, which he said was creating “further instability in Europe.

He said U.S. officials were still trying to assess the validity of the reports that North Korean troops are preparing to join the fight in Ukraine, the Associated Press reported.

“We are concerned by [the reports] … and we agreed to monitor the situation closely,” Mr. Campbell said.

Buryatia, Russia’s only Buddhist region, lies north of Mongolia in the Russian Far East. Remote and rarely visited by outsiders, it provides potential cover from prying eyes. Buryats are ethnic East Asians and so are — visually if not linguistically — impossible to differentiate from Koreans.

Buryatia, one of Russia’s poorest regions, has been a prime supplier of troops for the Russian army, which has suffered massive losses in the stalemated war that began with the invasion of Ukraine in February 2022.

Buryats won an impressive martial reputation during World War II, fighting for the Soviet Union. Buryats fighting in Ukraine though, unlike Russia’s notorious Chechen troops, are not deployed in specialist, segregated units

According to the Washington-based Wilson Center, Buryats have suffered disproportionately high casualties in Ukraine.

Russia’s elite airborne units, known as the VDV, have been used as “fire brigades” in Ukraine, spearheading assaults and operating as mobile reserves, though they have reportedly suffered massive casualties. Three thousand North Korean soldiers, the figure cited in earlier reports this week, would be enough to outfit a full brigade of three battalions.

In the early stages of the war, Russia’s invading force was formed into “battalion tactical groups,” but those formations proved too small to operate effectively. Brigades are currently the basic combat unit of the war on both sides.

The impact of the North Korean contingent on the fighting is uncertain. Ten thousand troops would on paper be enough to man three full brigades.

“One North Korean brigade is hardly a game-changer, from a military point of view,” said Gastone Breccia, an Italian military historian at the University of Pavia, when asked about the original reports that 3,000 troops were being trained.

That does not mean that North Korean troops would be useless to Mr. Putin — to Mr. Kim.

“I am certain that Putin would welcome North Korean troops to buttress his numbers, his losses being so high,” added Douglas E. Nash, a retired U.S. Army colonel. “What I would be more interested in is whether Kim Jong-un is using this ploy to clean out his prisons.”

Both the Russian and Ukrainian armies are using convicts on the front line in a desperate search for manpower to refill the depleted ranks.

Mr. Putin has conducted one mobilization of reservists, and is fighting the Ukraine war with contract soldiers (“kontraktniki’) as well as prisoners and private military companies to avoid deploying conscripts, particularly from the wealthy, elite cities like Moscow and St. Petersburg.

The use of soldiers from ethnic minorities and poor regions as cannon fodder is a long-held imperial practice, and Mr. Putin may be playing a similar game: The Ukrainian army says it has already captured ethnic Indians and Nepalese in Russian uniform.

“The old Soviet Union drew heavily upon ethnic minorities from [Central Asia] to fill its ranks,” said Mr. Nash, a military historian who has long studied World War II’s Eastern Front.

So large were their numbers that the German army formed “legions” of captured Soviet Armenians, Azerbaijanis, Kazakhs, Tatars and Turkmens, he added.