As world powers geared to meet Thursday in Vienna to mull the standoff with Iran, Washington said it was ready to enter the European-led negotiations if Tehran suspended uranium enrichment activities.

Washington's willingness to have its most substantive contacts with Iran since they broke diplomatic ties 26 years ago came amid efforts to hammer out an international approach to Iran's suspected nuclear arms ambitions.

A senior US official, who asked not to be named, linked the move to what he called an agreement by Russia and China to ease their categorical opposition to UN sanctions on Iran if negotiations stalled.

"What they have agreed is that if Iran does not accept this offer of negotiations or does not negotiate in good faith, we will return to the Security Council," the official said.

He said US Secretary of State Condoleezza Rice would finalize a list of possible sanctions to be imposed "if Iran rejects the offer of negotiations, does not suspend ... or does not negotiate in good faith."

But at least part of the price seemed to be conditional US agreement to participate directly in efforts by Britain, France and Germany to wean Iran off its nuclear activities with economic and other incentives.

"To underscore our commitment to a diplomatic solution and to enhance the prospects for success, as soon as Iran fully and verifiably suspends its enrichment and reprocessing activities, the United States will come to the table with our EU colleagues and meet with Iran's representatives," Rice said.

Washington had stayed on the sidelines of the talks but President George W. Bush told reporters Wednesday, "My decision today says the United States is going to take a leadership position in solving this issue."

They spoke hours before Rice left for Vienna to hold a crucial round of talks Thursday with her counterparts from Britain, France and Germany as well as Russia and China.

Analysts described the US announcement on the talks as a significant change in US policy towards a long-standing nemesis.

To drive home the message, Rice said US officials had conveyed her prepared statement to Iranian officials through the Swiss government, which handles US interests in Tehran, and Iran's UN mission.

The secretary also held a full blown news conference and half a dozen press interviews while aides went through their own Rolodexes of media contacts and opinion makers to promote her remarks.

There was no immediate response from Iran but a State Department official suggested the US offer on the negotiations was aimed more at the Europeans, Russia and China than Tehran.

"Even if Iran flips us off we will have shown our allies we have gone the extra mile for them," said the official, who spoke on condition of anonymity.

European states welcomed the new US stand. British Foreign Secretary Margaret Beckett said it would give "added weight" to the package of incentives world powers planned to offer Tehran this week.

"Direct US participation would be the strongest and most positive signal of our common wish to reach an agreement with Iran," said EU foreign policy chief Javier Solana.

Russian Foreign Minister Sergei Lavrov, visiting Turkey, did not address the US offer but said there was "a positive atmosphere" before the Vienna talks and added, "We hope the way will be open to a political solution."

But Rice made it clear the use of military force remained a possibility even while the olive branch was extended. "The president is not going to take any of his options off the table, temporarily or otherwise," she said.

The United States, which broke off ties with Iran after the 1979 seizure of US diplomats in Tehran, maintains channels through the Swiss and has had some direct contacts since then.

Their chief diplomats have occasionally rubbed elbows at diplomatic events. Iran and the United States took part in multinational talks on Afghanistan and agreed to meet directly to discuss Iraq but have yet to do so.

A senior US official, who asked not to be named, said talks on the nuclear issue would represent the first "meaningful contact with the Iranian government" since the hostage crisis.

But Rice stressed the talks would be focused on the nuclear question and not touch on broader issues that could lead to a normalization of relations.

"We're not talking about what some would characterize as a grand bargain," she said. "What we're talking about here is an effort to enhance the chances for a successful negotiated solution to the Iranian nuclear problem."

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