Last Monday, the chief U.N. nuclear inspector gathered ambassadors and experts from dozens of nations in a boardroom high above the Danube in Vienna, Austria, and laid out a trove of evidence that he said raised new questions about whether Iran had tried to design an atom bomb.

For more than two hours, representatives to the International Atomic Energy Agency were riveted by documents, sketches and even a video that appeared to have come from Iran's own military labs. The inspector, Olli Heinonen, said they showed work "not consistent with any application other than the development of a nuclear weapon," according to notes taken by diplomats.

The presentation caught no one's attention more than the Iranian representatives in the room, who deny Iran is developing atomic weapons. As they whipped out cell phone cameras to photograph the screen, Iran's ambassador, Ali Asghar Soltanieh, nearly shouting, called the evidence baseless fabrications, the diplomats said, and warned that the agency was going down "a very dangerous road."

Suddenly, the confrontation with Iran had reignited.

The display of new and newly declassified information is part of the latest effort to pressure Iran to disclose information about its past atomic activities and offer proof that its current program is benign. France's ambassador, Francois-Xavier Deniau, said questions raised by the Vienna meeting had opened a "new chapter" in the West's effort to keep Iran from acquiring nuclear arms, according to participants.

This confrontation is different than the long-running U.S.-led campaign. Gone are the veiled threats of military action from the White House. The wind largely went out of that effort in December when U.S. intelligence officials surprised Western allies - and angered Bush administration hawks - with a report saying that Iran had halted its nuclear weapons program in 2003. Last Monday's presentation in Vienna did not contradict that conclusion, but disclosed many new details suggesting the depth of Iran's past work on weapons design.

The new effort to pressure Iran has been led by Europeans and the international atomic agency. It will reach the U.N. Security Council today when talks begin on a possible third round of economic sanctions for Iran's continued refusal to stop enriching uranium for nuclear fuel.

The United States has been relegated to more of a behind-the-scenes role, largely because the December intelligence report left it with little leverage to continue confronting the Iranians.

But several allies said the report's conclusion was too sweeping, and President Bush has made clear that he shares that view.

For years, Washington had based its assessment that Iran was pursuing nuclear weapons largely on its steady work to enrich uranium, which could be used for bombs but which Iran says it wants to fuel power reactors. Forcing Iran to give up enrichment became the goal.

The December estimate, by contrast, focused on weapons design. Based on fresh intelligence that Iran's bomb design program was suspended in 2003, it said Iran was not currently pursuing nuclear weapons, even though uranium enrichment continued.

The Bush administration decided to let the atomic energy agency confront Iran with what it says is the best evidence of Iranian weapons work, some of which was revealed Feb. 25 in Vienna. The United States had previously shown some of that evidence to select countries, but it had declined to declassify all of the material.