South Korea demanded on Monday that the Russian ambassador protest military cooperation between Pyongyang and Moscow, calling for the immediate pullout of North Korean troops rumored to have been deployed to help Russia.

The Associated Press reported Friday that South Korea’s spy agency confirmed North Korea sent 1,500 special operations troops to help Russia in October in support of its war against Ukraine.

Ukrainian intelligence suggests North Korea was preparing to send 10,000 soldiers to join Russian forces, according to Ukraine President Volodymyr Zelenskyy.

The South Korean Foreign Ministry said Russian Ambassador Georgy Zinoviev met with Vice South Korean Foreign Minister Kim Hong-kyun, when the latter "condemned in the strongest terms" the North’s sending of troops to help Russia, which Kim said poses "a grave security threat" to South Korea and the international community.

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Kim also said South Korea and the international community will mobilize all available means to deal with the act, which threatens its vital national security interests, the foreign ministry added.

Zinoviev was quoted by the Russian Embassy as saying the cooperation between Russia and North Korea is not aimed at South Korea’s security interests.

NATO Secretary-General Mark Rutte spoke to South Korean President Yoon Suk Yeol over the phone on Monday, who said Seoul will not sit idly by as the "reckless" military cooperation between Moscow and Pyongyang continues. 

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Yoon also said South Korea plans to send a delegation to NATO to speak about Russian-North Korean cooperation, according to his office.

In a post on X, Rutte said North Korea’s decision to send troops to fight alongside Russia "would mark a significant escalation."

Pentagon press secretary Maj. Gen. Pat Ryder told reporters last week that the U.S. could not confirm or corroborate media reports on the North Korean troop dispatch to Russia.

Russia has previously denied using North Korean troops in the war, with presidential spokesperson Dmitry Peskov describing the claims as "another piece of fake news" during a news conference last week. North Korea's state media hasn't commented on the issue.

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North Korea and Russia, locked in separate confrontations with the West, have sharply boosted their cooperation in the past two years. The U.S., South Korea and their partners have accused North Korea of supplying artillery shells, missiles and other conventional arms to Russia to help fuel its war against Ukraine in return for economic and military assistance. In June, North Korean leader Kim Jong Un and Russian President Vladimir Putin signed a pact stipulating mutual military assistance if either country is attacked.

Many experts question how much the North Korean troop dispatch would help Russia, citing North Korea’s outdated equipment and shortages of battle experience. They say North Korea likely received Russian promises to provide it with high-tech weapons technology associated with its nuclear and missile programs, a move that will complicate U.S. and South Korean efforts to neutralize North Korean nuclear threats.

Ukrainian media reported this month that six North Koreans were among those killed after a Ukrainian missile strike in the partially occupied eastern Donetsk region on Oct. 3.

Many experts were previously skeptical of possible North Korean troop deployments to Russian-Ukraine battlefields because North Korea is preoccupied with its nuclear standoff with the U.S. and South Korea.

North Korea sent pilots to fight for North Vietnam during the Vietnam War and for Egypt during the Yom Kippur War in 1973, but there has been no known large-scale dispatch of its ground troops overseas.

The Associated Press contributed to this report.