VIENNA -- The U.N. nuclear agency on Thursday said it was worried Iran may currently be working on making a nuclear warhead, suggesting for the first time that Tehran had either resumed such work or never stopped at the time U.S. intelligence thought it did.

The report by the International Atomic Energy Agency appeared to put the U.N. nuclear monitor on the side of Germany, France, Britain and Israel. These nations and other U.S. allies have disputed the conclusions of a U.S. intelligence assessment published three years ago that said Tehran appeared to have suspended such work in 2003.

The U.S. assessment itself may be revised and is being looked at again by American intelligence agencies. While U.S. officials continue to say the 2007 conclusion was valid at the time, they have not ruled out the possibility that Tehran resumed such work after that.

Iran denies any interest in developing nuclear arms. But the confidential report, made available to The AP, said Iran's resistance to agency attempts to probe for signs of a nuclear cover-up "give rise to concerns about possible military dimensions to Iran's nuclear program."

Ali Asghar Soltanieh, Iran's envoy to the IAEA, told the official IRNA news agency that the report "verified the peaceful, nonmilitary nature of Iran's nuclear activities."

But U.S. State Department spokesman P.J. Crowley said the findings were consistent with what Secretary of State Hillary Clinton has been saying "on our ongoing concerns about Iran's activities."

The language of the report, the first written by Yukiya Amano, who became IAEA head in December, appeared to be more directly critical of Iran's refusal to cooperate with the IAEA than those compiled by his predecessor, Mohamed ElBaradei.

It strongly suggested intelligence supplied by the U.S., Israel and other IAEA member states on Iran's attempts to use the cover of a civilian nuclear program to move toward a weapons program was compelling.