President Barack Obama has unveiled a new nuclear policy that will limit the conditions under which the US can use nuclear weapons.
The policy demands that the US administration renounce the use of the weapons against non-nuclear countries, but still places exceptions for countries such as Iran and North Korea, which have violated or renounced the main treaty to halt nuclear proliferation.
For the first time the policy moves the prevention of nuclear proliferation and nuclear terrorism to the top of the US nuclear agenda.
The US has also pledged not to conduct nuclear tests, develop new nuclear warheads or pursue new missions or capabilities for nuclear weapons, and will press for a further ratification of the Comprehensive Test Ban Treaty.
The nuclear policy, contained in a document called the Nuclear Posture Review, opens a nine-day nuclear diplomacy meeting involving 47 nations.
Obama is also expected to sign a new arms-control agreement with Russia, which, he believes, is a stepping stone toward more ambitious reductions in nuclear arsenals in the future.