Countries believed to have tested hydrogen bombs

It may take weeks or longer to confirm or contradict North Korea's claim of hydrogen bomb test, but if true it makes it the only country to have succeeded in almost 20 years.

If North Korea's claim of a successful hydrogen bomb test is true, it joins a small number of nations who have tested the weapon. In that case, North Korea would become the only country to have carried out the test in almost 20 years.

The United States, South Korea and Japan agreed to launch a "united and strong" international response to North Korea's claim of a successful hydrogen bomb test, as experts scrambled Thursday to find more details about the detonation that drew worldwide skepticism and condemnation.

Watch: US, S. Korea and Japan vow tough response to N. Korea

It may take weeks or longer to confirm or contradict the North's claim, but a successful test would mark a major and unanticipated advance for its still-limited nuclear arsenal and push its scientists and engineers closer to their goal of building a warhead small enough to place on a missile that can reach the US mainland.

Countries believed to have tested hydrogen bombs

According to www.armscontrol.org, since the first nuclear test explosion on July 16, 1945, at least eight nations have detonated 2,053 nuclear test explosions at dozens of test sites from Lop Nor in China, to the atolls of the Pacific, to Nevada, to Algeria where France conducted its first nuclear device, to western Australia where the UK exploded nuclear weapons, the South Atlantic, to Semipalatinsk in Kazakhstan, across Russia, and elsewhere.

Graphic courtesy: Washington Post

The United States, Britain, France, Russia (as the Soviet Union) and China are known to have conducted hydrogen weapon tests. All these nations are signatories to the Non-Proliferation Treaty (NPT), an agreement that seeks to limit the spread of nuclear weapons.

In the five decades between that fateful day in 1945 and the opening for signature of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in 1996, over 2,000 nuclear tests were carried out all over the world.

The United States conducted 1,032 tests between 1945 and 1992.

The Soviet Union carried out 715 tests between 1949 and 1990.

The United Kingdom carried out 45 tests between 1952 and 1991.

France carried out 210 tests between 1960 and 1996.

China carried out 45 tests between 1964 and 1996.

The first hydrogen bomb

On 1 November 1952 the United States became the first country to test a hydrogen bomb. The Castle Bravo test on 1 March 1954 yielded 15 megatons and was the largest nuclear weapon ever detonated by the United States. By accident, the inhabited atolls of Rongelap, Rongerik and Utirik were contaminated with radioactive fallout, as was the Japanese fishing trawler Lucky Dragon. The controversy over radioactive fallout from testing activities caused great international concern.

The Democratic People's Republic of Korea announced that it had conducted a nuclear test in 2006, one in 2009 and again in 2013.