She also foreshadowed the results of a review being conducted by the Pentagon, which she said would produce a “transformational document” on the role, size and composition of the United States’ nuclear stockpile.

Mrs. Clinton hinted at broad changes in thinking since the last review in 2001. “Now is the time for fresh views on the role of the U.S. nuclear weapons arsenal,” she said. “We can’t afford to continue relying on recycled cold-war thinking.”

Her comments seemed calculated to shield President Obama from criticism by some in Congress who contend that his call for deep reductions in nuclear stockpiles could jeopardize the country’s defenses.

“We must do more than reduce the numbers of our nuclear weapons,” she said. “We must also reduce the role they play in our security.”

In her speech on the administration’s nonproliferation efforts, an address the State Department had billed as particularly important, Mrs. Clinton sought to impart a sense of urgency.

“Unless we act decisively and act now, the situation may deteriorate catastrophically and irreversibly,” she said in the speech, delivered to the United States Institute of Peace, a government-financed research center, on the occasion of its 25th anniversary.

Her remarks on Iran were particularly timely. Earlier in the day, during talks in Vienna, Tehran tentatively agreed to ship much of its stockpile of enriched uranium to Russia.

She welcomed the offer as “a constructive beginning” but cautioned that the United States would not put up with any Iranian tactics that seemed intended to buy time.

Similarly, while welcoming North Korea’s return to six-nation talks about dismantling the country’s nuclear program, Mrs. Clinton said that any improvement of relations would depend on credible actions by the North Koreans. No normalization of ties was possible with a nuclear North Korea, she said.