Washington (CNN) -- The United States will swear off the development of new generations of nuclear weapons and will not use its existing arsenal to attack nonnuclear states that are in compliance with nonproliferation agreements, the Obama administration said Tuesday.
The shift, a consequence of the government's Nuclear Posture Review, reflects the administration's changing view of the role of nuclear weapons in a post-Cold War world struggling to control the spread of potential devastating military technology.
Among other things, the new American stance is meant to provide an incentive for countries to stay within the rules of the 1968 Nuclear Non-Proliferation Treaty, a senior administration official said.
Defense Secretary Robert Gates, Secretary of State Hillary Clinton, Energy Secretary Steven Chu and Adm. Mike Mullen, the Joint Chiefs of Staff chairman, announced the change two days before President Obama is to sign a new nuclear arms treaty with Russia that reduces both countries' missile stockpiles.
The new policy "recognizes that the greatest threat to U.S. and global security is no longer a nuclear exchange between nations, but nuclear terrorism by violent extremists and nuclear proliferation to an increasing number of states," Obama said later in a statement.
"Moreover, it recognizes that 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."
Obama stressed that "preventing nuclear proliferation and nuclear terrorism is now at the top of America's nuclear agenda."
He noted the United States will not conduct nuclear testing and the administration will seek ratification of the 1996 Comprehensive Test Ban Treaty.
The position "provides a road map" to help achieve Obama's "long-term goal of a nuclear-free world," Gates added. It removes a "calculated ambiguity" in past U.S. nuclear policy while making clear that "this is a weapon of last resort," he said.
Gates also noted, however, the new policy sends a "strong message" to states such as Iran and North Korea.
"If you're going to play by the rules [of the nonproliferation treaty], 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."
Gates made clear that if a nonnuclear state uses chemical or biological weapons, it could still be subjected to a massive conventional response. He also warned the United States "reserves the right to make any adjustment to this policy" warranted by the future development of biological weapons.
Gates said Tuesday he had requested $5 billion to support a "credible modernization program" for the current U.S. nuclear infrastructure.
The new nuclear strategy calls for a halt to the development of new nuclear weapons and for an extension of the life of existing U.S. warheads.
In response to Obama's announcement, Arizona's two Republican senators, John McCain and Jon Kyl, issued a joint statement questioning the size of the government's desired financial investment in the new policy and the wisdom of limiting options for responding to attacks against the United States.
"The amount of money committed to this ... is woefully inadequate to bring our Manhattan Project-era facilities up to date and do the work necessary to enhance the reliability and extend the life of our warheads, all while maintaining the current stockpile," the statement by McCain and Kyl said. "This funding insufficiency must be corrected."
McCain and Kyl also said the new strategy confuses long-standing U.S. policy on retaining all options to respond to an attack by any state using weapons of mass destruction.
"The Obama administration must clarify that we will take no option off the table to deter attacks against the American people and our allies," the senators said.
A Pentagon fact sheet Tuesday said the U.S. government "would only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners."
In 2009, the Bulletin of Atomic Scientists estimated the U.S. nuclear stockpile at 5,200 nuclear bombs, including 2,200 operational, long-range strategic weapons.
The new treaty to be signed Thursday by the United States and Russia will limit each country to 1,550 strategic warheads, 700 deployed strategic delivery vehicles and 800 deployed and nondeployed strategic launchers, according to the Pentagon.
All U.S. Intercontinental Ballistic Missiles will be limited to a single warhead, according the Pentagon, "to increase stability."
U.N. Secretary-General Ban Ki-moon praised the administration's announcement, calling it a reaffirmation of Obama's commitment toward a "nuclear-free world."
Seven other nations -- Britain, France, Russia, China, India, Pakistan and North Korea -- are declared nuclear powers.
Israel is widely believed to have developed nuclear weapons but has never declared so publicly.
Obama has made reducing the global nuclear threat a top priority of his presidency and will host a summit joining 46 nations on that topic next week, the White House announced.
Obama will also hold bilateral talks with some of the visiting leaders, including Chinese President Hu Jintao, during the two-day summit that begins Monday in Washington, White House Press Secretary Robert Gibbs said.
Other bilateral meetings will be with the leaders of Armenia, Germany, India, Jordan, Malaysia, Pakistan, South Africa and Kazakhstan, Gibbs said.
In addition, the United Nations, the European Union and the International Atomic Energy Agency will be represented at the summit, Gibbs said.
CNN's Dan Lothian and Alan Silverleib contributed to this report.