Iran accepted a demand Thursday at the talks in a villa outside Geneva to allow U.N. inspectors into its covertly built enrichment plant, in a move that appeared to defuse tensions that had been building for weeks.
Western officials at the session said the Islamic republic had also agreed to allow Russia to take some of its enriched uranium and enrich it to higher levels for its research reactor in Tehran, a potentially significant move that would show greater flexibility by both sides.
U.S. President Barack Obama noted the deal in comments on the meeting. But Mehdi Saffare, Iran's ambassador to Britain, and a member of the Iranian delegation at the talks told The Associated Press the issue had "not been discussed yet." Asked if Iran had accepted, he replied: "No, no!"
Iran could use the agreement to argue that the six nations trying to engage it on its nuclear program acknowledge its right to enrich uranium, at the very low levels used for nuclear fuel of a reactor network it plans to build. The U.S., Britain, China, France, Germany and Russia remain formally committed to seeking a full freeze of Tehran's nuclear activities.
This is a key international goal because enrichment can produce both fuel for nuclear reactor and weapons-grade uranium for warheads.
But such agreement would also serve nations fearing that Iran is interested in ultimately further enriching its stockpile into weapons-grade uranium for warheads. Reprocessing the fuel for the research reactor would use up most of Iran's domestic stockpile of low enriched uranium, which Iran has amassed in sufficient amounts to build a bomb.
Speaking in Washington, Obama called the talks "a constructive beginning" and said Iran must match its words with action.
Tehran "must grant unfettered access" to international inspectors within two weeks, he said, warning that if Iran fails to follow through, "then the United States will not continue to negotiate indefinitely and we are prepared to move towards increased pressure."
"Our patience is not unlimited," Obama said. "Going forward, we expect to see swift action."
The tone of Thursday's meeting was considerably more positive than just a week ago, when the U.S. and its allies were threatening Iran with tough new sanctions if it refused to freeze its nuclear activities, which they suspect are aimed at creating an atomic weapon.
Perhaps the most significant development of the day was a 45-minute one-on-one meeting between U.S. Under Secretary of State William Burns and Iran's senior nuclear negotiator, Saaed Jalili. It was the first direct U.S. negotiations with Iran since Washington severed relations in 1980.
The encounter appeared to add to the positive atmosphere that led to agreement by all the parties _ Iran, the U.S., Britain, France, Russia, China and Germany _ for a follow-up meeting this month.
It also appeared to be concrete proof of Obama's commitment to engage Iran directly on nuclear and other issues _ a sharp break from policy under the Bush administration.
U.S. Deputy State Department spokesman Robert Wood said Burns used the meeting with Jalili "to reiterate the international community's concerns about Iran's nuclear program."
"He addressed the need for Iran to take concrete and practical steps that ... will build international confidence in the exclusively peaceful nature of its program," he said.
Wood said both sides also "had a frank exchange on other issues, including human rights." Officials in Washington said Burns urged Tehran to resolve the cases of three Americans detained in Iran since July.
Javier Solana, the European Union's foreign policy chief, told reporters Iran agreed to "cooperate fully" with the International Atomic Energy Agency and to open its newly disclosed nuclear facility to inspectors, probably within "a couple of weeks."
In a statement, the IAEA said agency chief Mohamed ElBaradei had been invited to Iran to discuss nuclear issues. A senior U.S. official said ElBaradei would travel to Tehran this weekend. He spoke on condition of anonymity because his information was confidential.
ElBaradei recently said Tehran was "on the wrong side of the law" over its new enrichment plant near the Shiite holy city of Qom. He said Iran should have revealed its plans as soon as it decided to build the facility.
Jalili reiterated Iranian claims that the disclosure was well ahead of time, but the two sides in the talks differed most strikingly in their interpretation of the plan to have an outside country further process enriched uranium for Iran. Tehran has long refused such an arrangement, insisting it has the right to a full domestic enrichment program.
Obama said such a step would help build international confidence.
"We support Iran's right to peaceful nuclear power," Obama said. "Taking the step of transferring its low-enriched uranium to a third country would be a step toward building confidence that Iran's program is in fact peaceful."
The differences reflected the likelihood of huge bumps ahead in any future talks.
Secretary of State Hillary Rodham Clinton, speaking in Washington, sounded a pragmatic note. "Today's meeting opened the door, but let's see what happens," she said.
Iran's refusal to freeze its enrichment activities has already prompted three sets of U.N. Security Council sanctions.
Iran came to the talks with a proposal that ignored the key demand that it freeze enrichment. Instead it offered to hold "comprehensive, all-encompassing and constructive" discussions on a range of security issues, including global nuclear disarmament.
Reiterating calls by President Mahmoud Ahmadinejad, the Iranian offer linked any talks with a discussion of Middle East tensions "to help the people of Palestine achieve all-embracing peace."
It called for "reform" of the U.N. Security Council _ shorthand for curbing the authority of the U.S. and the four other permanent council members. The only link to the arms issue was a call for discussion of disarmament by the world's nuclear powers.
Jalili told reporters that while those issues were key, Thursday's discussions were "good talks" compared to the last seven-nation meeting 15 months ago that broke up in failure.
At the United Nations, Iranian Foreign Minister Manouchehr Mottaki suggested the talks could be expanded to the "summit" level. He said Iran was willing to discuss a variety of security, economic and political issues, although he did not specifically refer to nuclear issues.
Associated Press writers Alexander G. Higgins, Bradley S. Klapper and Scheherezade Faramarzi in Geneva, John Heilprin at the United Nations, and Robert Burns in Washington contributed to this report.