Nine countries currently either say they have nuclear weapons or are believed to possess them.

The first to have nuclear arms were the five original nuclear weapons states — the United States, Russia, China, France and the United Kingdom.

All five are signatories to the Nuclear Nonproliferation Treaty, which commits countries that don’t have nuclear arms not to build or obtain them, and those that do to “pursue negotiations in good faith” aimed at nuclear disarmament.

Rivals India and Pakistan, which haven’t signed the NPT, have built up their nuclear arsenals over the years. India was the first to conduct a nuclear test in 1974, followed by another in 1998.

Pakistan followed with its own nuclear tests just a few weeks later.

Israel, which also hasn’t signed the NPT, has never acknowledged having nuclear weapons but is widely believed to have them.

North Korea joined the NPT in 1985 but announced its withdrawal from the treaty in 2003, citing what it called US aggression. Since 2006, it has conducted a string of nuclear tests.

Iran long has insisted its nuclear program is for peaceful purposes only and US intelligence agencies have assessed that Tehran is not actively pursuing the bomb now. But it has in recent years been enriching uranium to up to 60 percent purity — near weapons-grade levels of 90 percent.

In an annual assessment released this week, the Stockholm International Peace Research Institute estimated that the nine countries had the following stockpiles of military nuclear warheads as of January:

Russia: 4,309
United States: 3,700
China: 600
France: 290
United Kingdom: 225
India: 180
Pakistan: 170
Israel: 90
North Korea: 50