South Korea has summoned the Russian ambassador to Seoul to “strongly protest” against the alleged deployment of thousands of North Korean troops to support Russia in the war against Ukraine.

First Vice Foreign Minister Kim Hong-kyun told Russian envoy Georgy Zinoviev that the involvement of North Korean troops in the war violated UN resolutions and called for their immediate withdrawal, the South Korean Foreign Ministry said in a statement on Monday.

“We strongly condemn North Korea’s illegal military cooperation, including sending troops to Russia,” Kim said, adding that the operation posed a serious threat to the security of South Korea and other countries. “We will respond together with the international community by mobilizing all available resources against actions that threaten our core security interests.”

The decision to call up Zinoviev reflects growing anger in South Korea, Ukraine and the United States over the apparent use of North Korean troops under a secret military deal struck this year between Russian President Vladimir Putin and North Korean leader Kim Jong-un .

South Korea's national intelligence agency said last week that North Korea had sent an initial contingent of 1,500 special forces to Russia's Far East to train at local military bases, adding that they were expected to be active in the war in Ukraine would be used.

On Friday, the spy service said North Korea planned to send about 12,000 special forces to support Russia, possibly to make up for significant Russian troop losses and a lack of replacement troops.

Satellite images were also released showing the first deployment of elite North Korean soldiers being transferred to Vladivostok by Russian military ships between October 8 and 13.

“North Korea transported its special forces to Russia via a Russian Navy transport ship, confirming the beginning of North Korea’s military involvement,” it said.

The White House could not confirm reports that North Korean troops were fighting for Russia, but a National Security Council spokesman said if true, it would “mark a dangerous development in Russia's war against Ukraine.”

Ukrainian officials released a video on Monday purporting to show dozens of North Korean recruits lining up to collect Russian military uniforms.

The video released by the Center for Strategic Communications and Information Security – part of Ukraine's Ministry of Culture and Information – appears to show soldiers waiting to collect bags, clothing and other items from Russian military personnel.

“We received this video from our own sources,” Ihor Solovey, the center's director, told the Associated Press, adding that he could not provide additional confirmation for security reasons.

“The video clearly shows North Korean citizens receiving Russian uniforms at the direction of the Russian military,” he said. “For Ukraine, this video is important because it is the first video evidence showing that North Korea is participating in the war on the side of Russia. Now not only with weapons and grenades, but also with personnel.”

The center claimed the recordings were made by a Russian soldier in recent days. It was not said how the clip was obtained and the location is unknown.

Putin and Kim have not revealed details of their deal, which they agreed to during the Russian president's visit to Pyongyang in June. The countries also denied that North Korea had supplied Russia with ammunition and missiles.

Those denials were disputed by Ukrainian, South Korean and U.S. officials. On Friday, South Korean intelligence said North Korea had delivered more than 13,000 containers of artillery shells, rockets, anti-tank missiles and other deadly weapons to Russia since August.

Ukrainian President Volodymyr Zelensky said North Korea's involvement in the war would “significantly increase” the level of threat and instability in the region.

During his late-night video address on Sunday, Zelensky said satellite and video evidence from North Korean troops proved that another country had actually entered the war and called for a decisive response from Kiev's allies.

“I am grateful to the leaders and representatives of states who do not close their eyes and speak openly about this cooperation in the interests of a larger war,” he said. “We expect a normal, honest and strong response from our partners in this regard.”

North Korea has not commented on reports that it is sending troops to fight alongside Russia.