The United States warned Iran it will not have much time to respond once it is offered an international package of rewards to encourage it to suspend uranium enrichment, suggesting that the window could soon close and be replaced by penalties.
"It really needs to be within weeks," U.S. Secretary of State Condoleezza Rice said in a TV interview, referring to the six-power package of perks or penalties aimed at halting Iran's enrichment activities.
In separate comments on a radio program, Rice suggested she was ready to meet her Iranian counterpart, Manouchehr Mottaki, if Tehran agreed to suspend the activity that can be used to make nuclear arms and negotiate the details of the deal.
The package agreed on Thursday carries the threat of U.N. sanctions if Tehran remains defiant over what the West calls a rogue nuclear program that could produce a bomb. The United States, in a major policy shift, conditionally agreed this week to join those talks. It would be the first major public negotiations between the two countries in more than 25 years.
Rice met with the foreign ministers from the European nations that led talks with Iran, which stalled last year. European Union foreign policy chief Javier Solana, Russia's foreign minister and a deputy Chinese foreign minister also attended.
Russia and China might join in any future talks with Iran. Both hold vetoes in the U.N. Security Council, and the United States needs their cooperation to seek sanctions or other harsh measures.
The formal offer of talks are expected to be made by France, Britain and Germany -- the three nations that previously negotiatiated with Tehran. A senior U.S. state department official said he expected Tehran would be invited to begin new negotiations "within a matter of days."
A short statement issued by foreign ministers from the six powers and the European Union did not mention economic sanctions, which the U.S. wants and Iran has tried hard to avoid.
The powers agreed privately, however, that Iran could face tough Security Council sanctions if it failed to give up unranium enrichment and other disputed nuclear activities, U.S. officials said.
U.S. Undersecretary of State Nicholas Burns called the meeting's outcome "a step forward in our quest to deny Iran nuclear weapons capability."
The U.S. intelligence director, meanwhile said Tehran could reach that status in as little as four years.
"This is a matter of assessment, we don't have a clear-cut knowledge," National Intelligence Director John Negroponte said in an interview. "But the estimate we have made is that some time between beginning of the next decade and the middle of the next decade they might be in a position to have a nuclear weapon."
Diplomats feared Iran would reject any offer of talks if the threat of sanctions was explicit, officials involved in the discussions said on condition of anonymity because the seven-party negotiations were private.
The foreign ministers' statement threatens unspecified "further steps" in the Security Council.
The group's statement contained no details of incentives Iran could be offered. Diplomats previously have said the package includes help to develop legitimate nuclear power plants and various economic benefits.
"We are prepared to resume negotiations should Iran resume suspension of all enrichment-related and reprocessing activities," as previously required by the U.N. nuclear watchdog agency, said British Foreign Secretary Margaret Beckett.
If Iran returns to the talks, "we would also suspend action in the Security Council," Beckett said.
The Security Council, which can levy mandatory global sanctions and support its mandates with military force, has been reviewing Iran's case for two months. Its permanent, veto-holding members have been at odds over the possibility of sanctions, with Russia and China opposed.
"At this crucial stage, it is very important that none of the sides involved in the situation makes any sharp movements that would create a threat to the real prospect of using the chance to reach agreement," ITAR-Tass quoted Russian Foreign Minister Sergey Lavrov as saying before talks began in Vienna.
Iran insists its nuclear work is peaceful and aimed at developing a new energy source.
Mottaki, Iran's foreign minister, welcomed the idea of direct talks but rebuffed the U.S. condition that Tehran must suspend uranium enrichment before talks can begin.
At the White House, President George W. Bush warned that the confrontation would go to the Security Council should Iran continue to enrich uranium.
"If they continue their obstinance, if they continue to say to the world, `We really don't care what your opinion is,' then the world is going to act in concert," Bush said.
Bush said he got a "positive response" in a telephone conversation Tuesday with Russian President Vladimir Putin, adding, "We expect Russia to participate in the United Nations Security Council. We'll see whether or not they agree to do that."
Bush also spoke about Iran on Thursday with Chinese President Hu Jintao. He revealed little about that conversation, saying, "They understood our strategy."
The shift in U.S. tactics was meant to offer the Iranians a last chance to avoid punishing sanctions and to let the United States assert that it was willing to exhaust every opportunity to resolve the Iranian impasse without force.