The chief U.S. delegate to the International Atomic Energy Agency said Monday that the change in administrations in Washington would be a good opportunity for Iran to enter new negotiations to end its uranium enrichment program.

But the diplomat, Gregory L. Schulte, warned that Iran should not expect drastic changes in the U.S. position from President-elect Barack Obama.

"The president-elect has also talked about the need for reinforced diplomacy, the need for direct tough diplomacy, and the need for that diplomacy to be sustained, to be backed by the prospect of economic sanctions and political isolation," he said.

Iranian President Mahmoud Ahmadinejad has already sent congratulations to Obama, the first time an Iranian leader has offered good wishes to a U.S. president-elect since the 1979 Islamic Revolution. Obama has also indicated his willingness to talk with leaders such as those in Iran, Syria and North Korea.

Schulte underlined that Obama has echoed President George W. Bush''s stance that a nuclear-armed Iran is unacceptable. But Schulte said he hoped Iranian leaders would view the change in leadership at the White House as a new chance.

"We''ll see if they take advantage of this — I mean, it would be good if they said, ''OK, now is an opportunity for us to get into negotiations and to take concrete steps that the world expects,'' to give us assurances of the peaceful nature of their nuclear program," Schulte told reporters.

The U.S., Britain and many other Western countries accuse Iran of using its nuclear program as a cover for weapons development — a claim denied by Tehran, which is under U.N. sanctions over its refusal to cease enriching uranium, an activity that can make both nuclear fuel or fissile warhead material.

With the new administration, Iran should not expect a drastic change in the U.S. position, said Gregory L. Schulte, who was in Berlin to meet with German officials before the International Atomic Energy Agency''s board of governors'' meeting in Vienna on Thursday and Friday.

Iran''s relationship with the Bush administration has been marked by conflict. On Friday, the government in Tehran said Bush was "dreaming" of excuses to provoke confrontation. The U.S. has not ruled out military action unless Iran stops enrichment and heeds other U.N. Security Council demands.

Intelligence assessments from Israel, which considers Iran to be its greatest threat and dismisses Tehran''s insistence that its nuclear program is meant for peaceful purposes, said weakening international pressure on Iran will embolden the government to make major strides next year toward developing a nuclear bomb.

The assessments from Israeli intelligence officials were obtained Monday by The Associated Press.

Delays in activating Iran''s nuclear reactor will not hold up its development of nuclear weapons, because Tehran''s main focus is enriching uranium, the officials said. The intelligence officials agreed to be interviewed only on condition of anonymity because the information is classified.