Thermonuclear bombs could reach many times more power than previous physical nuclear bombs. The new bomb whose existence was announced by the USSR could, unlike the American one, be dropped from an aircraft, which was an important advantage in practice.
On August 20, 1953, the Soviet newspaper Pravda published the news of the successful testing of the first thermonuclear bomb in the USSR. It was a device called RDS-6 and with a power of about 400 kilotons, which was tested about eight days before the mentioned announcement. The American newspaper carried the news on the front pages, because it was a new great threat to the Western world. Thermonuclear weapons could have many times more power than ordinary nuclear bombs based only on the physics of heavy atomic nuclei. Admittedly, the RDS-6 was not yet a bomb whose power could be increased indefinitely, but a device in which physics still provided most of the energy.
The United States had tested its first thermonuclear bomb the previous year (1952) in the Pacific, so the Americans were ahead of the Soviets. However, the new bomb whose existence was announced by the USSR could, unlike the American one, be dropped from aircraft, which was an important advantage in practice (American first thermonuclear bombs were so large that they looked more like industrial plants than classic air bombs).