"The Iranians miscalculated. They thought they could divide the United States from our European allies and Russia and China, but we've been able to craft a united coalition of all those countries," Burns told NBC television.

"They have to choose -- negotiations with us, or further action in the security council," Burns said.

He added: "Frankly, I think they're rather cornered and isolated. There are very few countries defending them and they're running out of options."

"We think this policy of patient diplomacy and crafting multilateral coalitions to tighten the pressure on them is going to work, and it's the right way to go clearly for the United States," Burns said.

Tehran is facing mounting international pressure to give a clear answer before the Group of Eight (G8) summit from July 15-17 on an offer of incentives from world powers in return for suspending uranium enrichment activities.

Iran said Sunday it will take until the second half of August to respond to the international package, which was drawn up by the five permanent Security Council members plus Germany and presented to Iran on June 6.

Tehran Plays Tough In Crunch Talks On Nuclear Crisis

The West will hold fresh talks with Iran this week over its contested nuclear plans, but hopes for a breakthrough appear slim after Tehran again rebuffed calls to respond to an offer to defuse the row.

Iran's top negotiator Ali Larijani will on Tuesday again meet European Union foreign policy chief Javier Solana, as well as officials from countries behind the international offer to prevent a further escalation of the crisis.

Solana, who presented the offer to Tehran last month, expressed hope for progress after a private dinner in Brussels last Thursday with Larijani, who has been on a tour of European capitals.

"It's a good start," said Solana's spokeswoman Cristina Gallach after the dinner, adding: "We have laid the groundwork for the meeting (Tuesday) which we are hoping will be positive."

Larijani -- who travelled to Spain and Switzerland over the weekend, and is due in Rome on Monday before heading back to Brussels -- called the talks with Solana "fruitful and constructive."

The international offer, presented to Tehran by Solana on June 6 on behalf of the five permanent UN Security Council members plus Germany, offers economic and other incentives in exchange for a pledge to suspend uranium enrichment.

The West, in particular the United States, wants Tehran to respond before a Group of Eight (G8) summit in Saint Petersburg on July 15-17, but hardline President Mahmud Ahmadinejad has said Tehran will not respond before August.

US Secretary of State Condoleezza Rice said last week that the United States and its allies were expecting "a substantive response from Iran before the middle of July."

And she warned that, "if, indeed, Iran is trying to stall, it's not going to work. The international community has said that we need to get an answer, an indication of where Iran is going with this."

But Iran has played down prospects of substantial progress, saying it sees the meeting as a chance to "resolve ambiguities" about details of the offer, underlining that it will not give a full response in Brussels.

And on Sunday it again dashed hopes of a rapid response to the offer by saying it will take until the second half of August to respond.

"They need to respond to the ambiguities we have identified," Foreign Minister Manouchehr Mottaki said in Tehran. "We will respond in the last week of Mordad," Mottaki added, referring to the Iranian month which ends August 22.

Solana's spokeswoman implicitly acknowledged that the EU does not expect such a response on Tuesday, when Solana will be joined by representatives from France, Britain, Germany and Russia.

The aim of the Brussels talks will be "to agree on the terms of the negotiations .... on who will negotiate and also on what each side has to do for these negotiations," she said.

In any case the coming week could well be crucial. After Tuesday's new talks between Larijani and Solana the foreign ministers of the six countries behind the offer to Tehran will meet in Paris Wednesday to discuss the situation.

The West has made it clear that a rejection of the offer by Tehran would relaunch debate at the UN Security Council over what further measures to take against Iran.

It is unclear exactly where this would lead, in particular since Russia and China, which have veto rights, have made it clear they oppose sanctions against Tehran.

The Islamic republic has insisted it is serious about defusing the nuclear standoff, but has so far indicated that it is unwilling to suspend its uranium enrichment activities.

Iran says it wants to enrich uranium only to make civilian reactor fuel, although the process can be extended to make nuclear weapons.

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