South Korea on Monday demanded the immediate pullout of North Korean troops allegedly deployed in Russia as it summoned the Russian ambassador to protest deepening military cooperation between Pyongyang and Moscow.

The request was made during a call between NATO Secretary-General Mark Rutte and South Korean President Yoon Suk Yeol. NATO and the U.S. earlier said they could not confirm reports from South Korea and Ukraine that as many as 10,000 troops from North Korea are being sent to Russia.

Ukrainian officials, including Kyiv's top diplomat, and South Korea's intelligence agency say North Korea is deploying thousands of troops to help Russia with its war against Ukraine.

North Korea has said it had found the remnants of a South Korean military drone and that any further "violation" of its territory would result in a "declaration of war." Tensions between the two sides of the peninsula have escalated since May when the North began flying balloons carrying garbage across the border to the South,

North Korea has shipped 1,500 special forces troops to Russia’s far east for training and acclimatizing at local military bases and will likely be deployed for combat in the war in Ukraine, South Korea’s spy agency said on Friday.