Factbox: Key elements of Obama's new nuclear strategy

(Reuters) - The Obama administration, kicking off an intensive week of nuclear diplomacy, unveiled a revamped policy on Tuesday restricting U.S. use of its atomic weapons stockpile.

A Nuclear Posture Report is required by Congress of every U.S. leader but President Barack Obama had set high expectations when he vowed last year to end "Cold War thinking" and setting out a vision for a world free of nuclear weapons.

Following are key elements of the review:

* The United States for the first time is forswearing use of atomic weapons against non-nuclear countries, a break with a Bush-era threat of nuclear retaliation in the event of a biological or chemical attack.

But the policy shift came with the caveat that countries will be spared a U.S. nuclear response only if they are in compliance with the Nuclear Non-Proliferation Treaty. That loophole means nuclear-defiant Iran and North Korea would not be protected.

* "The United States will not develop new nuclear warheads," the document stated. This appeared to rule out going ahead with Bush-era pursuit of low-yield "bunker buster" nuclear bombs to be able penetrate buried targets such as Iranian nuclear facilities.

* The strategy review called for increased investment in upgrading U.S. weapons infrastructure, saying this "will not only guarantee our stockpile, but facilitate further nuclear reductions." Arms control experts see potential for significant cuts in the U.S. stockpile by upgrading weapons laboratories to weed out older, ineffective warheads.

* The administration pledged to pursue further arms control with Russia beyond the new START pact, which Obama and Russian President Dmitry Medvedev will sign in Prague on Thursday and which will cut nuclear arsenals by a third. It said any new efforts would address not only strategic weapons but also non-strategic and non-deployed nuclear weapons. The administration also pledged to pursue high-level dialogue with Moscow and Beijing to promote stability and transparency.

* The review stopped short of declaring deterrence from nuclear attack to be the only purpose of the U.S. nuclear arsenal and did not renounce a first-strike option -- as arms control advocates had hoped. But it did set a longer-term objective of making deterrence the "sole" purpose of America's atomic weapons.

* The document declared that the United States would keep up efforts to reduce the role of nuclear weapons in national security strategy while strengthening conventional arms capabilities.

* The Obama administration pledged not to conduct any further nuclear testing and to seek long-delayed ratification of the Comprehensive Nuclear Test Ban Treaty. The treaty was rejected by the U.S. Senate in the 1990s and still faces big hurdles with lawmakers who believe it can be exploited by Iran and North Korea.

(Reporting by Matt Spetalnick in Washington; Editing by David Storey)