The United States assailed Iran on Friday for "lies" about its nuclear program and voiced unprecedented criticism of the U.N. atomic agency chief, suggesting he glossed over 18 years of deception that included enriching uranium (search) and processing plutonium.

"Questionable," U.S. envoy Kenneth Brill (search) said of a section of a report from International Atomic Energy Agency Director-General Mohamed ElBaradei (search), adding the agency found "no evidence" of an Iranian nuclear program.

"Disingenuous," replied ElBaradei to Brill's criticism.

The exchange between the American and the Egyptian reflected deep differences at the IAEA board meeting over whether to condemn Iran's past nuclear transgressions or focus on what major European nations say seems to be its newfound openness.

Diplomats described both the dispute between Brill and ElBaradei and the rift within the board as unprecedented in more than two decades of meetings by the 35-nation board. Later, the State Department sought to play down any rift.

After two days of failure, the board adjourned until Wednesday in hopes of finding a compromise. IAEA spokeswoman Melissa Fleming said the pause would allow for high-level talks in foreign capitals.

Addressing delegates, Brill criticized Iran for "violations and lies" by enriching uranium, processing small amounts of plutonium, and other activities that Washington says point to a weapons agenda.

"Iran systematically and deliberately deceived the IAEA and the international community about these issues for year after year after year," Brill said. The purpose, he said, was "the pursuit of nuclear weapons."

Such conduct "constitutes noncompliance with its safeguards obligations," he said, in language that indirectly accused Iran of violating the Nuclear Nonproliferation Treaty (search) — an act that normally results in U.N. Security Council involvement.

Brill suggested a statement in ElBaradei's report was "questionable" in saying there was no "evidence" that Iran had tried to build nuclear weapons. Brill said the proper wording should have been that there was no "proof."

A combative ElBaradei dismissed the criticism.

"Frankly, I find it disingenuous that this word 'evidence' has suddenly become a matter of disbelief," he told board members, in comments made available to reporters.

Citing Black's Law Dictionary, ElBaradei, a lawyer, quoted entries from the book to plead his case that "proof" and "evidence" may be used interchangeably.

He suggested that in at least one instance — the war in Iraq — the IAEA's credibility was "enhanced," and America's diminished, because there is still no sign of the nuclear weapons program that Washington accused Saddam Hussein of having.

"We reflect facts, as radar does, without partiality," ElBaradei said. "We do not jump to conclusions or make leaps of faith. We have not said that we have come to the conclusion that the Iranian program is exclusively for peaceful purposes, because we still have work to do."

In Washington, State Department spokesman Adam Ereli praised the IAEA and ElBaradei.

"There's no intention to impugn the credibility of the International Atomic Energy Agency (search) and the fine work that Director ElBaradei has done in putting together what is an important report on Iran's nuclear program," Ereli said.

He also suggested the United States was backing away from its insistence that the IAEA board refer Iran's record on nuclear activities to the Security Council, saying: "We continue to work with our friends to make sure that the IAEA board of governors take fully into account what Dr. ElBaradei reported about Iran's nuclear program."

Earlier, Iran submitted a letter to the board agreeing to open its nuclear programs to pervasive spot inspections, giving up attempts to wait until it saw the text of the resolution and approved its language.

Diplomats, who spoke on condition of anonymity, said Iran continued to insist it had the right to withdraw its pledge if the resolution made reference to Security Council involvement or contained other language it found unacceptable.

Such a move, however, would almost guarantee a strong resolution that might even meet U.S. wishes to have Iran declared in violation of safeguard agreements — triggering possible Security Council involvement.

Asked what links there were between a soft resolution and his country's acceptance of wider inspections as well as its decision to suspend uranium enrichment — both of which are board demands — chief Iranian delegate Ali Akbar Salehi said: "They all go together."

He suggested the United States was isolated on the board.

"We think that the American delegation — or the U.S. as a whole — is sort of a hostage to its own accusations," Salehi said. "And I think the majority of the board are looking forward to see that this ... is resolved peacefully."

A diplomat who spoke on condition of anonymity said only a few countries — Canada, Australia and Japan — supported the U.S. position.

Salehi suggested that Germany, France and Britain — the chief backers of a relatively soft resolution — had pledged to keep the issue from going to the Security Council if Iran continued to cooperate with IAEA efforts to probe its nuclear past and present.

The European Union said it welcomed Iran's "new attitude" but warned that further "significant breaches" of IAEA agreements, "or evidence of further concealment," would be met "by an appropriate and immediate response from the international community" — shorthand for Security Council involvement.

ElBaradei has said he wants a strongly worded report that nonetheless stops short of asking for Security Council involvement.