US Defense Secretary Robert Gates said Tuesday that unprecedented limits being placed on the US nuclear arsenal will not weaken US defenses and will send a "strong message" to Iran and North Korea to "play by the rules."Under President Barack Obama's plan, the United States would narrow the circumstances under which a nuclear weapon would be used. The plan 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.According to the new plan, the US 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."All options are on the table when it comes to countries in that category," Gates said.Obama also has stopped short of saying the United States never will be the first to launch a nuclear attack, as many arms control advocates want. Gates said the administration decided against limiting US 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 at a press conference at the Pentagon. But "we also recognize the real world we continue to live in."Gates was joined by other cabinet members in announcing the plan, including Secretary of State Hillary Rodham Clinton and Energy Secretary Steven Chu.Gates said that a new policy restricting the use of nuclear weapons did not apply to countries such as Iran and North Korea, which he said are pursuing nuclear weapons in defiance of the international community.He also said the United States is moving toward a policy of no first use of nuclear weapons but is not ready to devise one yet. Gates said he did not believe "we were far enough along the road" toward controlling nuclear weapons around the world to give up the right to pre-emptive strikes.Clinton said the redrawn policy shores up the US 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 US 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. UN 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."Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said he"wholly endorses" the plan and believes it includes effectivedeterrents.Congressional Democrats also praised the decision, while some Republicans said it could weaken the US defense capability.Rep. Buck McKeon, the top Republican on the House Armed ServicesCommittee, said the policy change could carry "clear consequences" forsecurity and said he was troubled by "some of the language andperceived signals embedded" in the policy.Meanwhile, PrimeMinister Binyamin Netanyahu decided Tuesday evening to accept Obama’s invitation to next week’s nuclear summit inWashington.Amid concerns that the trip would increase pressureon Israel to open its nuclear facilities to international inspectors,the Prime Minister’s Office stressed that the summit would focus onpreventing the spread of nuclear know-how to terrorist elements, andnot on the nuclear capabilities of specific countries.