Updated at 1:30 p.m. ET
The Obama administration is narrowing the circumstances under which the U.S. would use nuclear weapons, altering a decades-old policy that helped maintain the global balance of power during the tense days of the Cold War.
The administration revealed the new policy Tuesday in a document called a nuclear posture review, drafted after a year of deliberation led by the Pentagon in consultation with allied governments.
President Obama in a statement today called the new U.S. defense policy "a significant step forward" in reducing the nuclear weapon role in security strategy, which will ease the global threat posed by these arms.
At the same time, he said that the new strategy will maintain "a safe, secure and effective nuclear deterrent" for the United States as long as nuclear weapons exist in the world.
"Our national security and that of our allies and partners can be increasingly defended by America's unsurpassed conventional military capabilities and strong missile defenses," he said.
The move seems certain to provoke a partisan debate. It is just one in a series of White House initiatives limiting the role of atomic warheads in national defense, following President Barack Obama's pledge last year to move toward a nuclear-free world.
The document alters the role of nuclear weapons in defense policy by reducing the number of potential U.S. nuclear targets. It asserts -- with caveats -- that the United States would not use nuclear weapons to respond to a chemical or biological attack.
That assurance would only apply to countries that had signed the Nuclear Non-Proliferation Treaty and had met their obligations. Countries flouting the treaty remain a threat to U.S. security, the document says.
"The United States is therefore not prepared at the present time to adopt a universal policy that deterring nuclear attack is the sole purpose of nuclear weapons, but will work to establish conditions under which such a policy could be safely adopted," it says.
Defense Secretary Robert Gates said today that the NPR has "a very strong message for both Iran and North Korea."
"If you're going to play by the rules... we will undertake certain obligations to you," he said. "But if you're not going to play by the rules, all options are on the table in terms of how we deal with you."
The United States also said it might adjust the policy if biological weapons technology developed more catastrophic potential.
On Thursday, Obama and Russian President Dmitry Medvedev are scheduled to sign a new START treaty, a bilateral agreement that will cut the number of strategic warheads and missiles maintained by the world's two largest nuclear powers.
The new nuclear policy comes ahead of a conference next month in which Obama will urge other nations to fight the spread of nuclear weapons. He needs to show that the United States is willing to take steps of its own, but the Senate is unlikely to ratify the new treaty with Russia for months and probably longer.
Obama also is hosting some 40 world leaders in Washington next week for a strategy session on nuclear security.
On Tuesday night, the White House will host a screening of the documentary "Nuclear Tipping Point" (a trailer for the film is available here). Those who will be in attendance for the screening include: former Secretaries of State Colin Powell, Henry Kissinger and George Schultz, former Secretary of Defense William Perry, and former chairman of the Senate Armed Services Committee Sam Nunn.
The White House's nuclear initiatives are intended to encourage other nations to reduce their stockpiles of atomic weapons or forgo developing them, and the review is the first of its kind since 2001 and only the third since the end of the Cold War two decades ago.
Obama would commit not to use nuclear weapons against non-nuclear states that are in compliance with the Nuclear Nonproliferation Treaty, even if they attacked the United States with biological or chemical weapons. Those threats, he told The New York Times in an interview, could be deterred with "a series of graded options" -- a combination of old and newly designed conventional weapons.
The White House also plans to urge Russia to begin talks on adopting first-ever limits on shorter-range, tactical nuclear weapons, an arena in which Russia holds an advantage, said officials who spoke on condition of anonymity because they were not authorized to discuss details of the policy review prior to its release.
Moscow has shown little interest in cutting short-range nuclear arsenals, because the Russian military relies on tactical weapons to balance what it sees as a threat from NATO to the west and China to the east.
On Tuesday, Foreign Minister Sergey Lavrov said Russia shares Obama's goal of a nuclear-free world, but said other nations must join the disarmament process as well.
Lavrov also said Russia reserves the right to withdraw from the new START treaty if it decides a U.S. missile defense shield, now planned for Romania, threatens its security.
Associated Press writers Vladimir Isachenkov reported from Moscow, and Jim Heintz from Kazakhstan.