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Officials from six world powers meet with Iran in Switzerland on Thursday for nuclear talks. ((Dominic Favre/Reuters))

Iran has pledged to open its recently revealed uranium enrichment plant to UN inspectors, possibly in the next few weeks, according to a senior EU envoy involved in the negotiations.

Javier Solana, who formally headed the negotiations Thursday in Switzerland, said Iran and six world powers also agreed to a second round of talks regarding Tehran's contentious nuclear program.

Iranian officials met with permanent members of the UN Security Council and Germany in Genthod, northeast of Geneva, in an attempt to persuade Tehran to freeze its uranium enrichment program.

And in a surprise development, American and Iranian delegates reportedly held their first known one-on-one meeting in years.

U.S. President Barack Obama called the meetings a "constructive beginning."

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Iran's chief nuclear negotiator, Saeed Jalili, attends a meeting in Switzerland on Thursday on Iran's nuclear power program. ((Dominic Favre/Reuters))

However, he stressed again that Iran must grant inspectors open access within two weeks to a recently disclosed uranium enrichment facility.

"Talk is no substitute for action," Obama said in Washington. "Our patience is not unlimited."

U.S. Secretary of State Hillary Rodham Clinton said Thursday that the talks had "opened the door" to potential progress on clarifying Iran's nuclear ambitions.

But she was cautious in her assessment.

"It was a productive day but the proof of that has not yet come to fruition, so we'll wait and continue to press our point of view and see what Iran decides to do," Clinton said.

The U.S.-Iran meeting, the Iranian pledge to open the plant to inspectors, and the round of talks are seen as encouraging signs that the discussions might be more fruitful than originally believed.

Western officials were discussing the imposition of new sanctions on Iran after learning last week of a second uranium enrichment plant under construction.

Concerns about Iran nuclear program

Iran insists the program is peaceful and for energy purposes. The West fears the program may be geared toward producing weapons and has demanded the doors be open to the UN nuclear inspectors.

Uranium processing is a key aspect of any nuclear power program. The mineral needs to be enriched, whether to fuel a nuclear reactor producing electricity or to be turned into bomb-making material.

Solana was upbeat before Western delegates, a three-man Iranian negotiating team and representatives from Russia and China began the one-day closed talks.

Iran was expected to bring a broad range of geopolitical issues to the table, while the six powers were seeking to soften Iran's resistance to freezing its uranium enrichment program.

Pressure on Iran increases

There has been a spike in international pressure on Iran since the leaders of the United States, France and Britain announced last week they have disclosed intelligence information to the International Atomic Energy Agency that confirms an underground nuclear facility in Iran and have demanded an in-depth investigation.

The U.S.-Iran meeting was a positive sign for negotiators.

U.S. spokesman Robert Wood said U.S. Under Secretary of State William Burns met with Saeed Jalili, Tehran's chief negotiator, during a lunch break at Thursday's seven-nation talks in Geneva.

Two other Western diplomats were also briefed on the discussion, Wood said. He declined to elaborate on what was discussed.

The U.S. has signalled it is already contemplating new and tighter sanctions on Tehran, reflecting expectations that the talks may end in failure. But diplomats at UN headquarters in New York said there has been no discussion of a new sanctions resolution.

The sidebar meeting is the first known direct meeting between Washington and Tehran in about 30 years. Washington severed relations with Tehran in 1980 during a hostage crisis in the wake of Iran's Islamic Revolution.

Previous sanctions

The UN has imposed three previous sets of sanctions on Iran for pursuing its uranium enrichment program, which the Islamic country says is for civilian reactors but the international community worries could be used to make fuel for nuclear weapons.

The first sanctions in 2006 focused on banning trade in materials, equipment, goods and technology that could contribute to the nuclear program.

The sanctions were expanded in 2007 to include arms exports from Iran. In 2008, Iran was restricted from importing technologies that could be used for both civilian and military purposes.

With files from The Associated Press