Pyongyang -- with whom Moscow signed a mutual defense pact -- is already widely believed to be arming Russia for its invasion, but troops on the ground would mark a new escalation in the conflict.

Ukraine's President Volodymyr Zelensky warned North Korea could "soon" have as many as 12,000 soldiers on Russian soil, while US President Joe Biden slammed the deployment as "very dangerous."

"We believe that the DPRK has sent around 10,000 soldiers in total to train in eastern Russia that will probably augment Russian forces near Ukraine over the next several weeks," Deputy Pentagon Press Secretary Sabrina Singh told journalists, using an abbreviation for North Korea's official name.

Washington had previously put the number of North Korean troops in Russia at more than 3,000.

NATO chief Mark Rutte likewise called the troop deployment "a dangerous expansion of Russia's war" and "a sign of Putin's growing desperation."

Rutte noted more than 600,000 Russian soldiers have been killed or wounded since the conflict started in 2022, adding the Kremlin was unable to sustain the invasion without foreign support.

Despite the cost, Russia has been making steady territorial gains in Ukraine.

Moscow's army has advanced 478 square kilometers (184 square miles) into Ukrainian territory since the beginning of October, a record since the first weeks of the war, according to an AFP analysis of data from the Institute for the Study of War.

Those gains followed 477 and 459 square-kilometer advances in August and September, respectively, and come amid amid major shifts on the front line, in particular in eastern Ukraine around Pokrovsk.

- Growing threats -

Speaking in Brussels after a briefing with South Korean intelligence officials, Rutte said he could confirm that North Korean military units had been deployed in the field in Russia's western Kursk region.

Ukrainian troops launched a ground offensive in Kursk in August and control several hundred square kilometers of Russian territory.

"The deepening military cooperation between Russia and North Korea is a threat to both the Indo-Pacific and Euro-Atlantic security," Rutte told reporters in Brussels.

Experts have said that in return for the troops, North Korea is likely aiming to acquire military technology, ranging from surveillance satellites to submarines, plus possible security guarantees from Moscow.

EU chief Ursula von der Leyen also warned that North Korea sending troops "for the first time" represented "a significant escalation of the war against Ukraine and threatens global peace."

She made the comments after a phone call with South Korean President Yoon Suk Yeol, during which she assured the leader that "the EU's response to this development will center on cooperation with the Republic of Korea and other like-minded partners."

The United States likewise told China -- an ally of both Moscow and Pyongyang -- it should be "concerned about this destabilizing action by two of its neighbors, Russia and North Korea," State Department spokesman Matthew Miller told reporters.

Rutte, a former Dutch prime minister who took over the reins of NATO this month, called on Moscow and Pyongyang to "cease these actions immediately."

- 'Enemy understands strength' -

At a press conference in Iceland on Monday, Zelensky had warned that there were already around 3,000 North Korean soldiers on Russian land -- with four times that expected imminently.

"We think that they will have 12,000 soon," the Ukrainian leader added.

"This is an escalation. Sanctions alone are not enough. We need weapons and a clear plan to prevent North Korea's expanded involvement in the war in Europe," Zelensky's chief of staff Andriy Yermak said on social media Monday after Rutte's comments.

"Today, Russia brings in North Korea; next, it could broaden their engagement, and then other autocratic regimes may see that they can get away with this and come to fight against NATO," he warned.

"The enemy understands strength. Our allies have this strength."

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