Barack Obama is set to announce a new defense strategy that would reduce the circumstances in which the US would be prepared to use nuclear weapons.

It would rule out a nuclear response to attacks on the US involving biological, chemical or conventional weapons, the BBC reported.

Nor would the US use nuclear arms on non-nuclear states that comply with the nuclear non-proliferation treaty.

Obama said he would make exceptions for states deemed in violation of the treaty, naming Iran and North Korea.

The details of his plan - the Nuclear Posture Review - are to be published later on Tuesday. Its release comes ahead of a planned signing by Obama and his Russian counterpart, Dmitry Medvedev, of a new nuclear arms reduction pact in Prague on Thursday.

The pact, agreed last month, commits Russia and the US to big cuts in nuclear warheads.

The pact would replace the 1991 Strategic Arms Reduction Treaty (Start), which expired last December.

The new treaty restricts both Russia and the US to 1 550 warheads, about 30% less than currently allowed, the US says.