Seoul, South Korea - South Korea urged Russia to stop its "illegal cooperation" with Pyongyang and voiced "grave concern" Friday as Moscow moved to ratify its defense treaty with North Korea.

On Thursday, Russian lawmakers voted unanimously to ratify a defense treaty with North Korea that provides for "mutual assistance" if either party faces aggression. It will now be sent to the upper Federation Council for approval.

South Korea and the US said that thousands of North Korean troops are training in Russia, with Ukraine saying this week that soldiers have arrived in the "combat zone" in Russia's Kursk border region.

Seoul "expresses grave concern over Russia's ratification of the Russia-North Korea treaty amidst the ongoing deployment of North Korean troops to Russia," the foreign ministry said in a statement.

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It added that the South Korean government "strongly urges the immediate withdrawal of North Korean troops and the cessation of illegal cooperation."

Seoul said it would work with allies to "take appropriate measures" over the move, and the country – a major arms exporter – has suggested it could revise longstanding policy barring it from sending weapons directly to Kyiv.

Russian President Vladimir Putin said in an interview aired Friday on state television that it was up to Moscow how it uses the treaty's clause on mutual military assistance.

"What action we take with this clause – that's still under question. We are in touch with our North Korean friends," Putin said.

"I mean to say that it's our sovereign decision, whether we use something or not – where, how – whether we need this, or we, for example, only carry out some exercises, training, passing on some experience – that's our business," he added.

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Seoul and Washington have long claimed that the nuclear-armed North is sending major shipments of arms to Russia.

One of North Korea's United Nations representatives said at the UN General Assembly's First Committee on Disarmament and International Security that the country was sending neither weapons nor soldiers to help Moscow.

The claims by South Korea and others are "nothing more than groundless rumors aimed at tarnishing the image of DPRK," Rim Mu Song said, referring to the North by its official name.

"It is yet another smear campaign devised by Ukraine" to get "more weaponry and financial support from the US and Western countries."

South Korea's representative flagged videos circulating online of North Korean soldiers in Russian uniforms speaking Korean, but Rim said they "again totally reject the allegation" of troop deployment.

South Korean President Yoon Suk Yeol has called the deployment a "provocation that threatens global security beyond the Korean Peninsula and Europe."

Yoon also said South Korea will "review" its stance on providing weapons to Ukraine in its war with Russia, which the country has long resisted.

North Korea amended its constitution to define the South as a "hostile" state and last week blew up roads and railways that once connected the two countries.