In a surprise move, Iran invited all 15 UN Security Council members to dinner Thursday in New York, yielding one of the highest-level US-Iran contacts since the 1979 Islamic Revolution.

The contact was with Iranian Foreign Minister Manouchehr Mottaki, who hosted an evening that was unexpected as it came in the midst of an escalating diplomatic crisis.

The five permanent Security Council members -- Britain, China, France, Russia and the United States -- are considering whether to impose a fourth round of sanctions on Iran over its nuclear ambitions.

They are trying to get Iran to stop enriching uranium, which can be used as fuel either for civilian power reactors or atomic weapons.

Washington and Tehran have had no diplomatic relations since April 1980, and face-to-face encounters between the countries' senior officials are rare.

In a recent breakthrough in contacts between Washington and Tehran, the number three from the US State Department William Burns met last October 1 in Geneva with the secretary of Iran's Supreme National Security Council, Saeed Jalili.

The United States is pursuing a policy of engagement with Iran but says this is part of a dual track approach that will include coercive measures such as sanctions if the Islamic Republic does not cooperate.

A British official told AFP: "We're always happy to talk. It's a dual-track process. We'll see what they've got to say."

Japanese ambassador Yukio Takasu said after the dinner: "We are not here to negotiate. We are here to exchange frank, general ideas. There were no talks about sanctions."

"What's important is that most Security Council members were here," Takasu told reporters in Japanese.

Washington saw the dinner "as another opportunity for Iran to show the council that it is prepared to meet its obligations and play by international rules," a US official told AFP, speaking on condition of anonymity.

"We'll make sure this fact is understood by all members so that this is not an opportunity for obfuscation by Iranian officials," the official said.

The dinner "is also an indication that the Iranians recognize that the efforts in the Security Council and elsewhere to get them to live up to their obligations are isolating them from the rest of the world and doing them harm," the official added.

The diplomats filed into the Iranian ambassador's residence on upscale Fifth Avenue in Manhattan at about 6:30 pm (2230 GMT). They left about two hours later.

China was the only one of the permanent council members to send its ambassador. The other four veto-wielding Security Council states had the number two or number three from their New York missions attending.

Austria, Bosnia and Herzegovina, Brazil, Japan, Lebanon, Mexico, Turkey and Uganda, all non-permanent members, sent their ambassadors.

Journalists saw delegations from 13 of the 15 Security Council nations. Diplomats from Nigeria and Gabon were not spotted.

Mottaki issued the invitation on the sidelines of the Non-Proliferation Treaty review at UN headquarters.

Washington has led the drive for three rounds of UN Security Council economic sanctions against Iran since December 2006.

Western powers suspect that Iran's civilian nuclear program is really aimed at producing nuclear weapons, a charge Tehran vehemently denies.

Iran has rejected a UN-drafted nuclear fuel swap deal which was meant to be a confidence-building gesture.

Iran would have shipped low-enriched uranium out of the country in return for fuel for a research reactor which makes medical isotopes.

The deal stalled after Iran insisted the two materials be exchanged simultaneously within its borders -- a condition the world powers reject.

Iranian President Mahmoud Ahmadinejad, who was at the NPT conference on Monday, charged that the United States was threatening Iran with nuclear weapons.

The United States, after a recent review of its nuclear policy, declared that it would not use nuclear weapons against non-nuclear states that are in compliance with the NPT.

But it did not rule out using them against states in non-compliance, like Iran which is defying UN sanctions.

In Brussels, US Vice President Joe Biden told European Union lawmakers Thursday that Iran "faces a stark choice: abide by international rules and rejoin the community of responsible nations, which we hope for, or face further consequences and increasing isolation."