President Barack Obama has unveiled a new nuclear policy that will limit the conditions under which the US can use nuclear weapons.

The new policy demands the US administration renounces the use of the deadly weapons against non-nuclear countries, but still places exceptions for countries such as Iran and North Korea, which have violated or renounced the international treaty to halt nuclear proliferation.

For the first time the US policy moves the prevention of nuclear proliferation and nuclear terrorism to the top of the nuclear agenda, and will see the country aligning its policies and funding with programmes to prevent the spread of nuclear weapons.

Promoting international responsibility in meeting non-proliferation treaty obligations is also a key aspect of the policy.

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 further ratification of the Comprehensive Nuclear 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.