Warning Tehran to stay in line, Mohamed ElBaradei, head of the International Atomic Energy Agency, said the measure sends an "ominous message that failures in the future will not be tolerated."
"This is a good day for peace ... and nonproliferation," ElBaradei told reporters, saying the resolution "strengthens my hand in ensuring that Iran's program is for peaceful purposes."
The text, adopted by the 35-nation board of governors of the Vienna-based International Atomic Energy Agency, was weaker than the United States had sought.
It avoided a direct mention of the Security Council — which has the power of imposing U.N. sanctions — to allow for compromise between the U.S. administration and key European powers seeking weaker wording.
ElBaradei nonetheless emphasized that the resolution gave him expanded powers both in probing Iran's past for signs of nuclear arms ambitions and supervising present programs to ensure they are peaceful.
While the text does not directly invoke the Security Council, its wording means the Council could be asked to get involved should there be "serious failures in the future" by Iran, ElBaradei said.
Adopted by consensus, the resolution warns against "further serious Iranian failures," saying that could lead the board to consider actions allowed by its statute — shorthand for possible referral to the Security Council.
While welcoming Iran's "offer of active cooperation and openness" — including suspending uranium enrichment and agreeing to thorough inspections on IAEA demand — the measure calls for a "particularly robust verification system" to test Tehran's honesty.
Washington had insisted last week it would hold out for at least a threat of Security Council action over 18 years of clandestine activities by Iran that U.S. officials say point to a weapons program, including enrichment and plutonium processing.
But France, Germany and Britain opposed a direct Security Council threat, fearing Iran could backtrack on its cooperation and its commitment to clear up questions about its past if it were too strongly pressured.
Backing the three European countries, Russian Foreign Minister Igor Ivanov said in Warsaw, Poland, Wednesday that invoking the Council "would have further complicated an uneasy situation."
A senior U.S. official, who spoke to The Associated Press on condition of anonymity, said Washington was "pretty happy" with the compromise text.
"The U.S. is very skeptical that Iran has stopped its covert nuclear weapons program, and it's only a matter of time until this comes out" under the resolution giving the agency greater policing powers, the official said.
In a slap at the United States and its allies, an Iranian statement said the resolution offered only "marginal relief to the few hard-liners" on the board.
"Iran's nuclear program is exclusively peaceful and will remain peaceful," the statement said.
But ElBaradei described his agency's probe as a "work in progress," adding: "We still have a lot work to do before we can conclude that Iran's program is exclusively for peaceful purposes."
A new report on Iran will be ready in February for board perusal, he said.
U.S. envoy Kenneth Brill asserted that the resolution already found Iran in "noncompliance" — and therefore pulled the "trigger" needed for Security Council involvement.
"The board will not countenance further evasive maneuvers by Iran," Brill told the meeting.
Later, he described Iran as being at a "crossroads."
"They can decide to continue down the well-worn path of the past — almost 20 years of denial, deception and deceit — or they can turn toward the path of a new chapter, wherein they really do come clean," he told reporters.
The United States, the European Union and most other members of the 35-nation board would like a permanent stop to Iranian uranium enrichment, but Iranian envoy Ali Akbar Salehi said that would not happen.
"The decision lies in the hand of the Iranians to restart the project in the future, once they feel the time is appropriate," he told reporters, describing the enrichment stop as "temporary."