This photo taken June 25, 2014, shows an inert Minuteman 3 missile in a training launch tube at Minot Air Force Base, N.D. The base is tasked with maintaining 150 of the nuclear-tipped missiles spread out across the North Dakota countryside and keeping them ready to launch at a moment's notice as part of the US's nuclear defense strategy.© Charlie Riedel/AP This photo taken June 25, 2014, shows an inert Minuteman 3 missile in a training launch tube at Minot Air Force Base, N.D. The base is tasked with maintaining 150 of the nuclear-tipped missiles…Nine countries are known to possess nuclear weapons.

The United States, Russia, the United Kingdom, France, China, India, Pakistan, Israel and North Korea have among them approximately 15,850 nuclear weapons — 4,300 of them deployed with operational forces, according to the Stockholm International Peace Research Institute.

The institute, an independent organization whose research centers on global security, said 1,800 of those weapons are kept in a state of high operational alert.

The number of nuclear weapons in the world is declining, mainly because Russia and the United States are reducing their stockpiles. The two countries’ arsenals make up more than 90% of nuclear weapons globally, according to the institute.

The two nations have agreed to the Treaty on Measures for the Further Reduction and Limitation of Strategic Offensive Arms — the new START treaty that came into force in February 2011.

The United States, China, France, Russia, and Britain are the only legally recognized nuclear weapons states under the 1968 international Treaty on the Non-Proliferation of Nuclear Weapons.

The United States was the first to develop nuclear weapons in 1945, and North Korea was the last, in 2006, according to institute data.

The hydrogen bomb, or H-bomb, which North Korea claimed to have successfully tested Wednesday, was first tested successfully by the United States in 1952. The device, which was never dropped on any targets, was tested by the Soviet government the following year.

Robert Kelley, a nuclear weapons expert at the institute, said he doesn’t think any other states beyond the nine have nuclear weapons, or are working hard toward having them at present. Iran had conducted nuclear weapons research, according to a United Nations inspectors report, but it agreed to limit its nuclear program to peaceful purposes under an agreement reached last year with world powers.

Iran’s deal with the United States, Germany, Britain, China, Russia and France would lift international economic sanctions in exchange for Tehran’s compliance. Over the next 10-15 years, Iran is required to reduce its stockpile of enriched uranium and the number of centrifuges it has to produce highly enriched uranium, a key component of a nuclear weapon.

South Africa voluntarily abandoned its nuclear weapons program, which the Apartheid government had launched in response to its world isolation and fear of communism in 1974. The government decided to dismantle its atomic bombs in 1989, the year before Nelson Mandela was released from prison, former president F.W. de Klerk said.

Taiwan, which launched a nuclear weapons program in the 1960s, is not believed to have any.

Kelley said he is skeptical about North Korea’s claim that it had a hydrogen bomb because of the small explosive yield of the test.

Lee Cheol Woo, a South Korean lawmaker, also placed doubt on the North’s claim, saying South Korea’s spy agency estimated the explosive yield at 6 kilotons, a small fraction of the over 100 kilotons of explosive yield that intelligence officials say a hydrogen bomb would release, South Korea’s Yonhap news agency reported. Lee said Wednesday’s yield was similar to that of an atomic bomb test by North Korea in 2013, which had an estimated yield of 7.9 kilotons.

Kelley said a successful hydrogen bomb could have a yield much lower than 100 kilotons, although the yield of Wednesday’s test did not reach the threshold.

“All we know is there was a seismic event that was almost certainly a nuclear test. To start giving them credit for having an H-bomb is a little premature,” Kelley said.