Seoul is “strongly” urging the immediate withdrawal of North Korean troops from Russia.

South Korea has expressed “grave concern” over Russia's attempt to forge a defense pact with North Korea, which is accused of supplying troops to Moscow for possible use in its war in Ukraine.

Russia's lower house voted unanimously on Thursday to approve the treaty, which requires Moscow and Pyongyang to provide “mutual assistance” if either side is attacked. The country's upper house is expected to soon follow suit.

Seoul “strongly urges the immediate withdrawal of North Korean troops and an end to illegal cooperation,” South Korea's Foreign Ministry said in a statement on Friday.

“The government will work with the international community to respond decisively to military cooperation between Russia and North Korea and take appropriate measures as their military cooperation continues to develop,” the ministry said.

Seoul's denunciation comes a day after South Korean President Yoon Suk Yeol condemned the deployment of North Korean troops in Russia as a “provocation that threatens global security” and raised the possibility of arms supplies to Ukraine.

“If North Korea sends special forces to the war against Ukraine within the framework of Russian-North Korean cooperation, we will gradually support Ukraine and also consider and implement the measures necessary for security on the Korean Peninsula,” Yoon said at a joint press conference after talks with Poland President Andrzej Duda.

“Although we have maintained our policy of not directly supplying lethal weapons, we can reconsider our stance more flexibly depending on the extent of North Korean military activities,” Yoon said.

U.S. and South Korean officials said Wednesday they were aware of the presence of 3,000 North Korean troops at several locations in Russia.

Ukraine's military intelligence said Thursday that Russian-trained North Koreans were already stationed in Russia's Kursk region, which borders Ukraine and has been the scene of heavy fighting.

As the BRICS summit in Kazan ended on Thursday, Russian President Vladimir Putin dodged questions about satellite images that appeared to show North Korean troop movements.

Putin, who signed a “comprehensive strategic partnership” with North Korean leader Kim Jong Un during a visit to Pyongyang in June, told reporters it was “our business” how Moscow implements the mutual defense clause.

North Korea's mission to the United Nations in New York on Monday dismissed claims that it had sent troops to Russia as “baseless, stereotypical rumors.”