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 wouldbecome 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 2053 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 2000 nuclear tests were carried out all over the world.

The United States conducted 1032 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.

Here's a timeline of the events:
(Source: www.ctbto.org)

Russia (1990): Forty two underground explosions were conducted on the Arctic islands until 24 October 1990. Although the site accounts for only 25 percent of all Soviet testing the aggregate yield of tests at Novaya Zemlya is estimated at 273 Mt roughly 94 percent of the total yield for all Soviet tests.

UK (1991): On 26 November 1991 the last British nuclear test code-named Julin Bristol took place at the Nevada Test Site (renamed Nevada National Security Site in 2010) in the United States. The explosion had a yield below 20 kilotons and was part of the development of the most recent generation of UK nuclear warheads designed for the submarine-based Trident missiles.

United States (1992): The United States conducted more nuclear tests than all other countries combined. The 20 kilotons underground nuclear test which was conducted at the test site in Nevada on 23 September 1992 was the last of 1032 nuclear tests carried out by the country. The first U.S. test - Trinity - had been detonated 47 years earlier on 16 July 1945.

France (1996): On 27 January 1996 the last nuclear test explosion by France was conducted at the Moruroa and Fangataufa Atoll test site in the South Pacific. The underground explosion was equivalent to 120000 tonnes of conventional explosives six times the force of the bomb dropped on Hiroshima in 1945.

China (1996): China conducted its last test on 29 July 1996 only two months prior to signing the Comprehensive Nuclear-Test-Ban Treaty (CTBT) on 24 September 1996.

Watch: The last nuclear test from China in 1996

After the CTBT was opened for signature in September 1996 about half a dozen nuclear tests have been conducted:

India conducted two tests in 1998 (India had also conducted one so-called peaceful nuclear explosion in 1974.) Pakistan conducted two tests in 1998. The Democratic People's Republic of Korea announced that it had conducted a nuclear test in 2006 one in 2009 and again in 2013.

Source: www.ctbto.org