During the Korean War some U.S. pilots thought they heard Russian being spoken over enemy radios in the northwest corner of Korea which they called "MiG Alley". No one believed this because they thought the Soviet Union was just supplying North Korea with training and equipment to fight the war. But after the fall of the Soviet Union we have been able to get information on private Soviet agendas from that era. The Korean War was technically two wars that are combined into one. You have the war on the ground that was around the 38th Parallel that people say was similar to World War I. Then you have the second war that was in the sky with all the U.S. bomber planes and jets (B-29 "Superfortress bombers and F-86 "Sabre" jets) versus the Soviet fighter planes (MiG-15 fighters). The U.S. was trying to cut the North Korean and Chinese ground forces from getting supplies. This all was happening over the place they called "MiG Alley". "MiG Alley is between Chang Ju and Huichon. After October 23, 1951, American B-29s would no longer go out in the day and eventually were not allowed to fly through "MiG Alley". The Soviet MiG-15s must have done a great job at stopping the American bombers. On April 21, 1952, during one of the battles three Soviet pilots were shot down and killed as they were preparing to land at one of the bases called Andong airbase.

So long story short, the Soviets seemed to be more involved with helping North Korea (and China) in the air force part of the war. There were Soviets from Russia involved in the air not only in training the North Koreans (and Chinese) but also helping in the fight. They did enter combat directly with the Americans in "MiG Alley and it seemed that they did a very good job at stopping thos B-29 bombers from bombing their supply line.

I hope this answers your question. I am new to this so I hope I did well. This question was a very good one. I have never thought of something like this and was very excited to do some research on it. Below is a link to the source to the article I got my information from. If you would like to know more than what I have given I suggest to read it, it was very interesting. If you need help with understanding anything it talks about I will be happy to interpret or assist with the text.

Soviet Involvement in the Korean War: A New View from the Soviet-era Archives