It's somewhat more complicated given that much of the significant work passed on by Klaus Fuchs to the Soviets that they acknowledge was responsible for the first Soviet fission bomb was Fuchs own work .. he shared with the British, the Americans, the Canadians, and the Soviets .. who were all ostensibly allies at the time.

https://en.wikipedia.org/wiki/Klaus_Fuchs

Adding to @eastbound's comment the Soviets were also responsible for the first remote operated "robot" on the moon.