Iran has agreed in principle to pull back the curtain on some of its most secretive nuclear research, U.N. officials said Tuesday, a concession that came hours before negotiators from the Islamic republic were due to begin crucial talks with six world powers on curbing its nuclear program.

The tentative agreement, announced in Vienna by U.N. nuclear officials, could give inspectors access to Iranian scientists and facilities long shielded from international scrutiny. The move was greeted with a mixture of optimism and wariness from Western diplomats seeking to discern whether it represented a genuine breakthrough or an attempt by Iran to gain an advantage before the start of Wednesday’s talks.

Officials from the United States and five other powers — Britain, China, France, Germany and Russia — are expected to press Iran to accept strict curbs on its nuclear activities during the negotiations, which many diplomats and security experts see as a last chance to stave off a military confrontation.

Despite more conciliatory signals from Tehran in recent weeks — culminating with the inspection deal announced Tuesday — it was unclear whether Iran would agree to any new restrictions on a nuclear program that it consistently has said is for peaceful purposes.

“We’re clear-eyed going into this,” said a senior Western diplomat, who spoke on the condition of anonymity to discuss diplomatically sensitive preparations for the talks, which were scheduled to last one day. “The signs from Iran so far have been positive and different from what we have seen before. But Iran needs to show a seriousness and a clear willingness to get on to the substance of the issues.”

Tuesday’s apparent accord between Iran and the U.N.’s International Atomic Energy Agency attempts to resolve one of the thorniest disputes between Iran and Western governments in recent years: the nation’s refusal to account for a secret program of alleged nuclear weapons research conducted as recently as 2003. Iran insists that it has never sought to manufacture nuclear weapons, but it has routinely blocked access to key scientists and to military installations where the work was alleged to have occurred.

After a previously unscheduled visit to Iran over the weekend, IAEA Director General Yukiya Amano said Tuesday that the two sides had essentially settled their differences and were formalizing a plan that would ease the investigation of Iran’s past nuclear activities, ending a six-year stalemate.

“I can say it will be signed quite soon,” Amano told reporters at the Vienna airport upon his return from Tehran. While a few obstacles remain, a “decision was made to conclude and sign the agreement,” he said.

In Washington, the Obama administration cautiously welcomed Amano’s announcement, but several officials noted that similar agreements had fallen apart when Iranian officials refused to provide the promised access.

“It’s an agreement in principle that represents a step in the right direction,” White House spokesman Jay Carney told reporters. “We will make judgments about Iran’s behavior based on actions.”

The Israeli government, which has threatened military strikes against Iran to stop what it sees as Tehran’s pursuit of nuclear weapons, was openly skeptical of the claim of a diplomatic breakthrough.

“The Iranians are trying to reach a ‘technical agreement,’ which will create the impression of progress in the talks,” Israeli Defense Minister Ehud Barak said at the start of a Ministry of Defense meeting. By appearing to make concessions, Iran is seeking merely to deflect international pressure on itself, he said.

Neither Iran nor the IAEA provided details of the accord, although Amano spoke of progress on a “structural agreement” that laid out the terms under which Iran would give the agency information about its past nuclear research.

Tehran’s chief nuclear negotiator, Saeed Jalili, spoke vaguely about what he said were “very good talks” with the U.N. nuclear agency. “God willing, we will have good cooperation in the future,” he added.

To some former U.S. officials and arms-control experts, the apparent progress at the Tehran meeting was a positive sign.

“Now the task is to reach agreement on specific, concrete proposals, followed by actions, that can help prevent a nuclear-armed Iran,” said Daryl Kimball, executive director of the Arms Control Association, a Washington-based nonprofit.

Western diplomats in recent days have publicly aired their views about what some of the proposals and actions should be. In interviews, some have outlined a multistage strategy in which Iran would be called upon to immediately halt some nuclear activities, including its production of a more highly purified form of enriched uranium that could be used to build weapons. Iran also is likely to be pressed to shut down its newest uranium-enrichment plant, which is built in mountain bunkers beyond the reach of most conventional bombs and missiles.

Significant relief from international sanctions would come later, after Iran carried out the initial “confidence building” measures and agreed to permanent curbs that would keep the nation from emerging as a nuclear weapons state.

Current and former Obama administration officials acknowledged that the chances for a comprehensive agreement Wednesday were slim, given the complexity of the issues and the time needed for consultations between the negotiators and their governments. But several officials said they expected at least to have firm indications from Iran about its willingness to address Western concerns.

“One doesn’t need to see a breakthrough in these talks — it’s not realistic,” said Dennis Ross, who until last fall was President Obama’s chief adviser on Iran. “But you need to see indicators that they are willing to talk about some of these things.”

Staff writer Karin Brulliard contributed to this report from Jerusalem.