Growing military cooperation between Russia and North Korea poses a "distinct threat and a grave challenge to the peace and security on the Korean peninsula and in Europe," South Korea's President Yoon Suk Yeol told Reuters on July 8.
Russian President Vladimir Putin visited Pyongyang on June 18-19, where he signed a partnership agreement with North Korean leader Kim Jong Un.
The two leaders led hours-long talks before signing the treaty, which pledged to provide aid to one another if either will be attacked.
"North Korea is clearly a menace to the international society," Yoon told Reuters. "I hope that Russia will sensibly decide which side - the South or the North - is more important and necessary for its own interests."
Yoon gave the comments ahead of his departure to the NATO summit in Washington, D.C., which will take place on July 9-11.
NATO Secretary General Jens Stoltenberg said on July 5 that the summit will discuss ways to increase cooperation between Ukraine and South Korea.
Seoul last month said it would consider supplying Ukraine with weapons in response to Moscow's recently announced security agreement with North Korea.
Up until now, Seoul has only provided humanitarian aid to Kyiv, though it has reportedly indirectly supplied artillery shells to Ukraine via the U.S. Meanwhile, Pyongyang has been supplying Moscow with ammunition for use against Ukraine.