WASHINGTON (AP) — President Barack Obama vowed Tuesday to constrain the use of the nation’s Cold War-era nuclear arsenal, part of a bold but politically risky move aimed at discouraging nuclear technology from spreading.

Obama’s plan, a sharp departure from his predecessor’s policy, is a bid to play down the threat posed by nations like Russia and China while emphasizing the threat posed by terrorists or states believed to encourage terror.

“To stop the spread of nuclear weapons, prevent nuclear terrorism, and pursue the day when these weapons do not exist, we will work aggressively to advance every element of our comprehensive agenda: to reduce arsenals, to secure vulnerable nuclear materials, and to strengthen” international agreement, Obama said in a statement.

Under the new plan, the U.S. promises not to use nuclear weapons against countries that do not have them. The policy would not apply to states like North Korea and Iran, however, because of their refusal to cooperate with other countries on nonproliferation standards.

Obama’s plan would lessen the role nuclear weapons play in America’s defense planning.

U.N. Secretary-General Ban Ki-moon said he welcomes the president’s reaffirmation of his commitment toward a nuclear-free world and believes the new Nuclear Posture Review “is a timely initiative in that direction.”

Congressional Democrats also praised the decision, while some Republicans said it could weaken the U.S. defense capability.

Rep. Buck McKeon, the top Republican on the House Armed Services Committee, said the policy change could carry “clear consequences” for security and said he was troubled by “some of the language and perceived signals embedded” in the policy.

At a Pentagon news conference, Defense Secretary Robert Gates said the unprecedented limits being placed on the U.S. nuclear arsenal will not weaken the nation’s defenses and will send a “strong message” to Iran and North Korea to “play by the rules.”

“All options are on the table when it comes to countries in that category,” Gates said.

Obama has stopped short of saying the U.S. will never be the first to launch a nuclear attack, as many arms control advocates want.

Gates said the administration decided against limiting the nation’s options further because of the danger still being posed by the proliferation of nuclear weapons.

“This is obviously a weapon of last resort,” Gates told reporters. But “we also recognize the real world we continue to live in.”

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said he “wholly endorses” the plan and believes it includes effective deterrents.

Secretary of State Hillary Rodham Clinton said the redrawn policy shores up the U.S. commitment to global nonproliferation efforts, including the Nuclear Non-Proliferation Treaty, under which states without nuclear weapons are supposed to refrain from developing them.

She said Washington is reinforcing its commitment to a nonproliferation culture “by stating clearly for the first time that the United States will not use or threaten to use nuclear weapons against non-nuclear weapon states” that have signed the treaty and abide by it.

Clinton said the U.S. will continue to try to seek common ground with Russia on the issue of missile defense despite the Kremlin’s suspicion that such systems are aimed at crippling its nuclear arsenal.

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