South Korea's foreign ministry voiced "grave concern" Friday after Russia moved to ratify its defence treaty with North Korea, calling again on Moscow to stop its "illegal cooperation" with Pyongyang.

On Thursday, Russian lawmakers voted unanimously to ratify a defence treaty with North Korea that provides for "mutual assistance" if either party faces aggression.

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

North Korea and Russia have previously denied the deployment.

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. 

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

"The government will work together with the international community to firmly respond to military cooperation between Russia and North Korea, and take appropriate measures as their military collaboration progresses," it said.

On Thursday, South Korean President Yoon Suk Yeol called the deployment a "provocation that threatens global security beyond the Korean Peninsula and Europe", after talks with Polish President Andrzej Duda.

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 citing longstanding domestic policy. 

North Korea has adopted a new national anthem, state media reported Friday, another move that experts suspect will further North Korean leader Kim Jong Un's drive to define his country as entirely separate from, and in conflict to, the South. 

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.