SEOUL, South Korea (AP) — South Korea demanded its immediate withdrawal Monday North Korean troops are said to have been deployed in Russia because it called on the Russian ambassador to protest against the deepening of military cooperation between Pyongyang and Moscow.

The South Korean spy agency said on Friday it had confirmed that North Korea had sent 1,500 special operations troops to Russia this month in support of Moscow’s war against Ukraine. Ukrainian President Volodymyr Zelensky previously said his government had this in place 10,000 North Korean soldiers were prepared to join invading Russian forces.

During a meeting with Russian Ambassador Georgy Zinoviev, South Korean Vice Foreign Minister Kim Hong Kyun condemned “in the strongest terms” North Korea’s troop deployment, which he said poses “a serious security threat” to South Korea and the international community. The ministry reported this in a statement.

Kim said South Korea, in cooperation with the international community, will mobilize all available resources to address any 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 against South Korea’s security interests.

In a telephone conversation with NATO Secretary General Mark Rutte, South Korean President Yoon Suk Yeol said on Monday that Seoul will not stand idly by over the “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, Yoon’s office said. Rutte wrote on X that North Korea possibly fighting alongside Russia would “mark a significant escalation.”

The US and NATO have not confirmed that North Korean troops have been sent to Russia. But reports of their presence have already fueled concerns in South Korea that Russia could provide North Korea with advanced technologies that could sharply improve the North’s nuclear and missile programs in return for the deployment of troops.

North Korea’s advancing nuclear arsenal poses a major threat to South Korea’s security. North Korean leader Kim Jong-un has recently taken steps to permanently seize power end all relationships with South Korea and threatened to use nuclear weapons preventive. Some observers say South Korea is likely to consider supplying weapons to Ukraine if Russia’s transfer of high-tech nuclear and missile technologies to North Korea is verified.

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

Russia has previously denied using North Korean troops in its war with Ukraine. North Korean state media have not yet commented on the matter. Ukrainian officials released a video reportedly shows North Korean soldiers lining up to pick up Russian military clothing and bags at an unknown location. The Associated Press could not independently verify the images.

Asked about North Korean forces during a conference call with reporters on Monday, Kremlin spokesman Dmitry Peskov said that “we see a lot of conflicting information.”

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

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

Cooperation between North Korea and Russia has blossomed over the past two years. The US, South Korea and their partners have accused North Korea of ​​delivery conventional weapons to Russia in exchange for economic and military aid. In June, Kim and Russian President Vladimir Putin signed an agreement pact establishing mutual military assistance if either country is attacked.

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