SEOUL, South Korea (AP) — South Korea called for immediate withdrawal Monday North Korean troops are said to be stationed in Russia when the Russian ambassador was summoned to protest against deepening military cooperation between Pyongyang and Moscow.

South Korean intelligence said Friday it had confirmed that North Korea had transmitted 1,500 special operations forces to Russia this month to support Moscow's war against Ukraine. Ukrainian President Volodymyr Zelensky previously said his government had intelligence information about it 10,000 North Korean soldiers were prepared to join the invading Russian forces.

During a meeting with Russian Ambassador Georgy Zinoviev, South Korean Deputy Foreign Minister Kim Hong Kyun “strongly condemned” North Korea's troop deployment as he said it posed a “serious security threat” to South Korea and the international community, the South Korean Foreign Ministry shared the ministry said in a statement.

Kim said South Korea, in cooperation with the international community, will mobilize all available means to deal with an act that threatens its vital national security interests, the statement said. The Russian embassy quoted Zinoviev as saying that Russian-North Korean cooperation is not directed against South Korea's security interests.

In a phone call with NATO Secretary General Mark Rutte on Monday, South Korean President Yoon Suk Yeol said Seoul would not stand idly by and watch “reckless” military cooperation between Pyongyang and Moscow. Yoon said South Korea will soon send a delegation to NATO to exchange information on Russian-North Korean cooperation, according to Yoon's office. Rutte wrote on X that a possible North Korean fight alongside Russia “would mean a significant escalation.”

The US and NATO have not confirmed the deployment of North Korean troops to Russia. But reports of their presence have already fueled fears in South Korea that Russia could provide North Korea with sophisticated technologies that could significantly improve the North's nuclear and missile programs in return for the troop deployment.

North Korea's advancing nuclear arsenal poses a major security threat to South Korea. North Korean leader Kim Jong Un recently initiated steps toward permanent abolition end all relationships with South Korea and threatened to use nuclear weapons preventive. Some observers believe that if Russian transfers of high-tech nuclear and missile technologies to North Korea are confirmed, South Korea will likely consider arms sales to Ukraine.

South Korea joined US-led sanctions against Moscow over its invasion of Ukraine in February 2022. But South Korea has not directly supplied Kiev with weapons, citing its longstanding policy of not supplying weapons to countries actively involved in conflict.

Russia has previously denied using North Korean troops in the war with Ukraine. North Korea's state media did not comment on the matter. Ukrainian officials released a video It purportedly shows North Korean soldiers lining up at an undisclosed location to collect Russian military clothing and bags. The Associated Press could not independently verify the footage.

Asked about the North Korean troops during a conference call with reporters on Monday, Kremlin spokesman Dmitry Peskov said: “We see a lot of contradictory information.”

“South Koreans say one thing, then the Pentagon says it has no confirmation of such statements. There is a lot of contradictory information,” Peskov said. “It needs to be treated as such.”

North Korea's deployment of troops to Russia would be North Korea's first involvement in a major war since the end of the 1950-53 Korean War. Many experts question how much North Korean troops would help Russia on the battlefield, citing their lack of combat experience.

Cooperation between North Korea and Russia has flourished in the past two years. The US, South Korea and their partners have filed charges North Korea delivers conventional weapons to Russia in return for economic and military aid. In June, Kim and Russian President Vladimir Putin signed a deal Pact that provided for mutual military aid if one of the countries is attacked.

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Associated Press writer Kim Tong-hyung contributed to this report.