Throughout the late 1940s, aware that thermonuclear weapons were developed by the United States, the Soviet Union worked to develop a hydrogen bomb to counter the perceived Cold War threat.

Soviet physicist Andrei Sakharov (considered the father of the Soviet H-bomb) formulated key ideas that contributed to Soviet thermonuclear design and his research played a key role in the development process.

 A year after the U.S. tested its first thermonuclear device in November 1952, the Soviets tested a thermonuclear bomb yielding ~400 kilotons of TNT at the Semipalatinsk Test Site in northeast Kazakhstan.

Although the test produced a significantly smaller yield than the 1952 American thermonuclear test series, the Soviets argued that this weapon was ready for immediate use and could be delivered by a long-range bomber

Soviet TU-4 Bomber – Wikipedia

Main source: Soviet Hydrogen bomb project