Iran made good on recent threats yesterday and announced that it will resume building equipment essential for a nuclear weapons program, despite its agreement with three major European powers.

The decision does not violate international treaties that allow Tehran to make centrifuge parts for peaceful nuclear energy. But the move does break an agreement Iran signed with France, Britain and Germany, in which it promised to suspend nuclear efforts as a goodwill gesture toward earning trade incentives with the European Union.

European officials and arms-control specialists called Iran's move a major setback and a reflection of the difficulties faced by those working to check Iran's nuclear ambitions as evidence mounts that the country is concealing information from international inspectors.

John R. Bolton, undersecretary of state for arms control, told Congress that Iran's move is a "thumb in the eye of the international community." Bolton said the United States is determined to take the matter soon to the U.N. Security Council.

Iranian officials, who threatened to restart nuclear programs in response to a harsh rebuke last week by the United Nations' nuclear watchdog agency, insisted their efforts remain peaceful.

"We assure European countries Iran is not searching for nuclear weapons, but we will never abandon nuclear technology and the mastery of this science," Ayatollah Ali Khamenei, Iran's top leader, said in a statement.

In a letter to the French, British and German foreign ministers, Iran said it will resume centrifuge production -- a move that would bring the country a significant step closer to making highly explosive nuclear material.

"All Iran would have to do now is put uranium into the centrifuges, and then they can start producing a key ingredient for nuclear weapons," said David Albright, a former nuclear inspector.

In diplomatic terms, Iran's defiance was seen as a direct challenge to the United States, which is trying to convince allies that it is time to punish Iran at the Security Council.

U.S. and European officials, working in concert, claimed a victory last week when the International Atomic Energy Agency's 35-member board censured Iran for failing to comply fully with agency inspectors trying to determine whether the country is hiding a weapons program.

The board asked Iran to stop all enrichment production and to reconsider design and construction of a heavy-water nuclear reactor. In the past 18 months, inspectors have uncovered an escalating series of contradictions in Iranian statements, along with evidence that nuclear specialists consider strongly suggestive of a clandestine nuclear weapons program, as the United States has asserted.

European allies do not disagree with the assessments but believe that diplomatic incentives could help persuade Iran to give up its nuclear ambitions. The Bush administration has taken a tougher line, but at the same time it has supported the Europeans in their approach.

Iran was initially responsive, and in April it halted centrifuge construction. But it has since accused Germany, Britain and France of reneging on their promise to help end the matter within the IAEA.

Yesterday, diplomats conceded that Iran's latest move had thrown their efforts into doubt.

"This is certainly a negative development," said one European diplomat, who acknowledged that the deal is now in serious jeopardy.

Albright, who revealed satellite images last week showing newly destroyed Iranian facilities, said: "The whole program to suspend ways to enrich uranium is unraveling, and unless it's put back together again, the international community will come under pressure to isolate Iran and impose economic sanctions."

In remarks to the House International Relations subcommittee on the Middle East, Bolton said the United States believes that Iran is also working on biological and chemical weapons programs.

The State Department is searching shipments to Iran for material that could be used to make weapons of mass destruction, Bolton said later yesterday to the American Enterprise Institute.