Senior diplomats from France, Britain and Germany are scheduled to meet with Iranian officials this week in London in an effort to salvage a diplomatic initiative that virtually fell apart last month when Iran announced it would resume building equipment that could be used for making a nuclear bomb, U.S. and European officials said yesterday.

Iran had pledged to halt activities consistent with a weapons program in exchange for trade incentives from the European Union. But it backed out of some terms in June after the Europeans supported a toughly worded rebuke of Iran for failing to cooperate with international inspectors.

It is unclear whether Iran will have anything new to offer at tomorrow's meeting or how the outcome will affect U.S. policy toward the country. The continued standoff and suspicion surrounding Iran's weapons capabilities has embarrassed the European trio, frustrated Washington and worried international nuclear inspectors.

Yesterday, the International Atomic Energy Agency confirmed that Iran is now rebuilding centrifuges and using parts that had been briefly under IAEA seal as part of Iran's private agreement with the Europeans. The work is being monitored by agency inspectors who have been investigating Iran's nuclear efforts.

The Bush administration, convinced the Iranians are concealing a weapons program, is hoping its European allies will take a tough approach at the upcoming meeting and offer Iran a last chance to suspend its nuclear programs or face international condemnation in the U.N. Security Council, a senior administration official said.

But European diplomats said they are committed to finding a diplomatic way out of the stalemate. "We're just going to sit with them and find out where we can go from here," said one European diplomat who agreed, on the condition of anonymity, to discuss strategy before the meeting.

The Europeans are eager to reach a determination about Iran's intentions before the International Atomic Energy Agency meets in Vienna in September to consider Tehran's cooperation with agency inspectors.

Frustrated by Iran's poor performance during the spring, the IAEA's 35-member board condemned Tehran in a June statement largely written by France, Britain and Germany. It also asked Iran to stop all enrichment production and to reconsider plans for a heavy-water nuclear reactor.

But the three European powers were surprised days later when Tehran responded by announcing that it would resume building equipment essential for a nuclear weapons program.

Under international treaties, Tehran is allowed to make centrifuges and other parts for peaceful nuclear energy. But 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.

The European allies do not disagree with the assessments but believe that diplomatic incentives could help persuade Iran to give up its nuclear ambitions. Iran was initially responsive to those efforts, and in April it briefly halted centrifuge construction.