The UN Security Council's five permanent members plus Germany were meeting in Paris on Wednesday to try to forge a united front in the international standoff over Iran's nuclear programme.
The talks -- involving foreign ministers of the six world powers -- come a day after a meeting between EU foreign policy envoy Javier Solana and Iran's top nuclear negotiator, described as "unsatisfactory" by EU officials.
US Secretary of State Condoleezza Rice, who flew into the French capital early on Wednesday, warned that Iran's response to a Western offer aimed at resolving the crisis so far appeared "disappointing and incomplete".
"I want to wait, to hear from Javier Solana first hand about how he sees the situation, but certainly the indications are that Iran's response has been disappointing and incomplete," she told reporters.
But she stressed that "negotiation is still the best path and negotiation is still open".
World powers are seeking a rapid response from Iran on a package of economic and political incentives designed to persuade it to freeze uranium enrichment -- a process needed to fuel a nuclear reactor but which could also be used to make a bomb.
Western powers have repeatedly warned that Iran would be exposing itself to Security Council action if it rejects the offer, made by the EU on June 6 and backed by the United States, Russia and China.
However both Russia and China -- which have UN Security Council veto rights -- have made it clear they oppose sanctions against Iran.