The meeting, slated to begin in a few hours, brings together foreign ministers from the five permanent members of the UN Security Council plus Germany, along with European Union foreign policy chief Javier Solana.
Ahead of the meeting, the United States offered to join multilateral talks with Iran if it stopped enriching uranium.
U.S. Secretary of State Condoleezza Rice announced the policy shift on May 31.
"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.
Frosty Iranian Reaction
In an initial response, Iran's official news agency (IRNA) described Rice's offer as a propaganda move.
Then Iranian Foreign Minister Manuchehr Mottaki said today that Iran was ready to hold talks with the United States, but he stressed that Iran would not give up what it has called its right to uranium enrichment.
"[Rice's] announcement contained nothing new, only old excerpts from her previous interviews and speeches," Mottaki said. "It showed no fresh or logical solution to the nuclear issue."
He dismissed the offer as "rambling" aimed at covering up what he described as U.S. "crimes" in the region.
Brussels, Beijing Welcome Overture
Some of the participants in the planned talks today in Vienna have already publicly praised the U.S. initiative.
UN High Representative for Common Foreign and Security Policy Solana said it represented "the strongest and most positive signal of our common wish to reach an agreement with Iran."
Beijing, too, welcomed the move -- but with reservations. China's envoy to the UN, Wang Guangya, urged Washington to "not have any preconditions" for talks.
Onus On Iran?
Still, Washington hopes the offer now puts it at a diplomatic advantage by placing the onus for the continuing nuclear crisis on Tehran -- with Iranians facing a choice between "two clear paths," in Rice's words.
The United States might hope that will build support for Western efforts to get a new and binding UN resolution demanding Tehran stop uranium enrichment or risk unspecified punitive measures.
"There is an important diplomatic component because the U.S. can't be seen as always the rejectionist party, because that makes it difficult for the other [permanent member states of the Security Council] to go along with what the U.S. is proposing in terms of punitive sanctions," nuclear expert Shannon Kile of the Stockholm Peace Research Institute in Sweden told RFE/RL.
Washington is reported to already have made some concessions to the Russian position to in a bid to get Russian support.
U.S. media quoted officials as saying privately that Washington has accepted language for the proposed binding Security Council resolution that makes no mention of the possible use of force to make Iran comply with it.
That would be in line with Moscow's and Beijing's insistence that the Iranian nuclear crisis be solved only through diplomatic means.
(with additional agency reports)