Obama unveils new nuclear strategy

Review bars nuclear retaliation against biological or chemical attacks.

President Barack Obama's administration has announced a major shift in US nuclear weapons strategy that includes ruling out their use in response to attacks on America involving biological, chemical or conventional weapons.

But the Nuclear Posture Review also warned that countries such as North Korea and Iran could still be targeted if they fail to comply with the Nuclear Non-Proliferation Treaty.

"[Tehran and Pyongyang's] continued defiance of international norms and agreements will lead only to their further isolation and increasing international pressure," the document notes.

It also includes a pledge by the US not to develop any new weapons and outlines plans for "achieving substantial further nuclear force reductions" beyond the new treaty.

Publication of the review comes two days before Obama signs a landmark nuclear weapons treaty with Russia. The pact, agreed last month, commits Russia and America to large reductions in nuclear warheads and is designed to replace the 1991 Strategic Arms Reduction Treaty (Start), which expired last December.