TEHRAN/WASHINGTON (Reuters) - Iran said on Tuesday it would not discuss a previously secret nuclear plant at international talks this week but Washington vowed to bring it up and demanded Tehran prove it is not developing an atomic weapon.

White House spokesman Robert Gibbs, asked about Iran's insistence it would not discuss the facility in the Geneva talks, declared: "They may not, but we will."

Iranian officials and representatives of six major powers, including the United States, China and Russia, will hold talks on Tehran's nuclear ambitions in Geneva on Thursday. It is the first such encounter since U.S. President Barack Obama took office early this year promising more active U.S. diplomacy.

Ali Akbar Salehi, head of Iran's Atomic Energy Organization, made clear that Iran feels the newly disclosed uranium enrichment plant is off-limits for discussion.

"We are not going to discuss anything related to our nuclear rights, but we can discuss about disarmament, we can discuss about non-proliferation and other general issues," Salehi told a news conference.

"The new site is part of our rights and there is no need to discuss it," he said, adding Tehran would not abandon its nuclear activities "even for a second."

The back-and-forth suggested a tense atmosphere and little optimism ahead for the talks, after U.S. President Barack Obama joined with leaders of Britain and France last week to disclose the existence of the Iranian plant and call on Tehran to let the International Atomic Energy Agency inspect it.

Iran's IRNA news agency quoted MP Mohammad Karamirad, a conservative and member of parliament's foreign policy and national security commission, as saying Iran could close the door completely to cooperation with world nuclear authorities.

POSSIBLE NEW SANCTIONS

"If the Zionists and America continue their pressure on Iran and if the talks...do not reach a conclusion, then parliament will take a clear and transparent position, such as Iran's withdrawal from the NPT," he said.

Washington has suggested possible new sanctions on banking and the oil and gas industry if Tehran fails to assuage Western fears it seeks nuclear weapons. U.S. officials believe sanctions could now have more effect, playing on leadership divisions evident since a disputed Iranian presidential poll.

Gibbs, at a White House briefing, said the onus was on Iran "to demonstrate visibly for the world that they have a peaceful nuclear program designed for power and energy rather than a secret program to develop a nuclear weapon."

The United States and its Western allies have made clear they will focus on Iran's nuclear program at the Geneva meeting. Iran has offered wide-ranging security talks.

"My expectation, or my hope, is that we will be able to get...the guarantees from Tehran, that the program in which they are engaged in is a peaceful program," EU foreign policy chief Javier Solana told reporters in Gothenburg, Sweden.

"I don't think it will be easy to ask for, but we will continue to engage