President Obama is due to sign a new nuclear pact with Russia this week
President Barack Obama's administration has unveiled a defence policy to significantly narrow the circumstances in which the US would use nuclear arms.
But its Nuclear Posture Review warned that countries breaking the rules would remain potential targets.
The US strategy document also raised concerns about a "lack of transparency" in China's nuclear programme.
The review comes two days before Mr Obama and his Russian counterpart sign a landmark nuclear arms reduction pact.
The deal, agreed last month, commits Russia and America to big cuts in nuclear warheads and is to replace the 1991 Strategic Arms Reduction Treaty (Start), which expired last December.
Kim Ghattas, BBC News, Washington
The document is carefully worded - it limits the use of nuclear weapons but carves out exceptions to use them against countries that break the rules - in other words countries like North Korea and Iran.
This new nuclear policy reflects a changing world - while nuclear weapons were useful to achieve a balance of power with countries like Russia or China, the modern threats require a more nimble defence strategy.
President Obama though is likely to be criticised by both sides- the left will say he should impose a blanket ban on the use of American nuclear warheads.
The right will say he's undermining the credibility of America's nuclear deterrent by limiting the circumstances in which it can be used.
The far-reaching Nuclear Posture Review, published on Tuesday, outlines plans for "achieving substantial further nuclear force reductions" beyond the new treaty.
Every president since 1991 conducts such a review - the last one took place in 2001 at the start of George W Bush's administration.
But Mr Obama set high expectations when he declared in Prague last year America's commitment to seek a world without nuclear weapons.
For the first time, the US is ruling out a nuclear response to attacks on America involving biological, chemical or conventional weapons.
But this comes with a big caveat: countries will only be spared a US nuclear response if they comply with the Nuclear Non-Proliferation Treaty - this does not include Iran and North Korea.
"[Tehran and Pyongyang's] continued defiance of international norms and agreements will lead only to their further isolation and increasing international pressure," the document notes.
Tehran insists its nuclear programme is peaceful, but its refusal to adhere to international demands has raised fears of a possible strike on its nuclear facilities by the US or Israel.
Defence Secretary Robert Gates outlined the Nuclear Posture Review at a Pentagon briefing on Tuesday and said it would pave the way to a nuclear-free world.
The lack of transparency surrounding China's nuclear programmes raises questions about its strategic intentions
The document said America would only use nuclear arms in "extreme circumstances", and committed it to not developing any new nuclear warheads.
But the review also said the US would maintain its conventional arsenal to reassure its allies.
The review said China's nuclear arsenal remained much smaller than those of Russia and America.
"But the lack of transparency surrounding its nuclear programmes - their pace and scope, as well as the strategy and doctrine that guides them - raises questions about China's future strategic intentions," it noted.
The White House announced later on Tuesday that Mr Obama would hold talks with Chinese President Hu Jintao on the sidelines of a nuclear non-proliferation summit next week.
Dozens of world leaders are to attend the two-day conference, which begins in Washington on Monday.
Mr Obama won the Nobel Peace Prize last year, in part for his vision of a nuclear-free world.
Hillary Clinton: US nuclear weapons play "a stabilising role"
The BBC's Kim Ghattas, in Washington, says the new US nuclear policy reflects a changing world.
While nuclear weapons have been useful to achieve a balance of power with countries like Russia or China, modern threats require a more nimble defence strategy, our correspondent says.
But the Obama administration's strategy review is still likely to be criticised by both sides of the political divide, she adds.
The new nuclear pact - which Mr Obama is due to sign on Thursday in Prague with Russian President Dmitry Medvedev - commits the former Cold War rivals to significant cuts in their weapons' stockpiles.
The treaty would restrict both Moscow and Washington to a maximum of 1,550 warheads each, about 30% less than currently allowed, the US says.
But Russia drew some lines in the sand on Tuesday, warning it could pull out if it decided a US missile defence shield, proposed for Europe, threatened its security.
All numbers are estimates because exact numbers are top secret.
Strategic nuclear warheads are designed to target cities, missile locations and military headquarters as part of a strategic plan.
Israeli authorities have never confirmed or denied the country has nuclear weapons.
The highly secretive state claims it has nuclear weapons, but there is no information in the public domain that proves this.
The International Atomic Energy Agency reported in 2003 there had been covert nuclear activity to make fissile material and continues to monitor Tehran's nuclear programme.
US officials have claimed it is covertly seeking nuclear weapons.
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