But many other countries, including the U.S., have known stockpiles of nuclear weapons.
More than two dozen nations have nuclear power. Only nine possess actual nuclear weapons: Russia, the United States, China, India, Israel, France, North Korea, Pakistan and the United Kingdom.
The Ploughshares Fund, a global security foundation, estimates there are more than 15,000 nuclear weapons around the world; the U.S. and Russia possess 93 percent of them. The former Cold War foes keep nearly 2,000 nuclear weapons at the ready for immediate launch against each other, according to the Nuclear Threat Initiative.
While the exact number in each country's arsenal is often a closely guarded secret, some information is publicly available. Here's a breakdown of nuclear arsenals by country, based on data from leading experts in nuclear estimates.
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Total nuclear weapons: Unknown. The U.S. said in February it had intelligence indicating the secretive nation could soon have enough plutonium for nuclear weapons and was taking steps toward a long-range missile system, but experts do not believe North Korea currently has the technology to deliver weapons.