The 5 NWS parties have made undertakings not to use their nuclear weapons against a non-NWS party except in response to a nuclear attack, or a conventional attack in alliance with an Nuclear Weapons State. However, these undertakings have not been incorporated formally into the treaty, and the exact detail of them has at times varied. (The United States, for instance, has indicated that it may use nuclear weapons in response to an attack with non-nuclear "weapons of mass destruction", such as chemical or biological weapons.)
Several states refuse to sign the treaty. India, Pakistan, and Israel possess nuclear weapons, which would be prohibited by the treaty. South Africa formerly undertook a nuclear weapons program, with the assistance of Israel, and may have detonated a nuclear test over the Atlantic, but has since renounced its nuclear program and signed the treaty in the early 1990s after destroying its small nuclear arsenal. India and Pakistan have publicly announced possession of nuclear weapons, and have detonated nuclear tests. Israel has allegedly been developing nuclear weapons at its Dimona site in the Negev since 1958, and is believed to have stockpiled hundreds of warheads—claims the Israeli government refuses to confirm or deny. North Korea signed, but revoked its signature after a dispute with inspectors over inspections of non-declared nuclear facilities.