Secretary of State Hillary Clinton pledged Tuesday that the United States would maintain a "stabilizing role" for its allies after issuing a new policy restricting the use of nuclear weapons.

"For generations, the United States' nuclear deterrent has helped prevent proliferation by providing our non-nuclear allies in NATO, the Pacific, and elsewhere with reassurance and security," Clinton told reporters.

"The policies outlined in this review will allow us to continue that stabilizing role."

The United States has been committed since the aftermath of World War II to providing a nuclear umbrella for key allies including Japan, South Korea and members of the North Atlantic Treaty Organization (NATO).

In a new policy, the United States committed not to use nuclear weapons against a non-nuclear state in compliance with the Non-Proliferation Treaty.

Defense Secretary Robert Gates said NATO would decide the alliance's nuclear posture but that it would maintain a nuclear component.

"As long as there are nuclear weapons that threaten NATO, NATO will need to maintain a nuclear capability," Gates said.

Clinton said it would be up to NATO as a whole to determine any changes to its nuclear posture.

"We make it very clear that any changes in NATO's nuclear posture should only be taken after a thorough review within and decision by the alliance," she said.

The United States has never renounced the "first use" of nuclear weapons, and Obama's policy stops short of calls by arms control activists to explicitly limit their role to deterrence of other nuclear-armed states or terror groups.

Clinton later told diplomats from a number of countries meeting at the State Department that the "threat of nuclear terrorism is unfortunately and sadly very real.

"We see it all the time as there are attempts by terrorist networks to obtain access to nuclear radiological materials... literally around the world," Clinton said.

"Any authority in any country must stand against the threat of nuclear terrorism fomented by extremist networks who have no conscience and no concern for life."