Major powers have given Iran two weeks to answer calls to rein in its nuclear programme or face tougher sanctions after talks ended in stalemate despite unprecedented participation from the United States.
A US State Department spokesman said Washington hoped Iran now understood that it had a choice between cooperation and "confrontation, which can only lead to further isolation".
But prospects of ending a row that has triggered regional tensions and rattled oil markets looked dim as Iran's top nuclear negotiator insisted Tehran would not even discuss a demand to freeze uranium enrichment at the next meeting.
"We still didn't get the answer we were looking for," European Union foreign policy chief Javier Solana said on Saturday after six hours of talks in Geneva with Iran's Saeed Jalili and envoys from the US, Russia, China, France, Germany and Britain.
Mr Solana said he hoped for a clear answer from Tehran in about two weeks to a month-old sextet offer of trade and technical incentives to halt enrichment.
Asked whether Tehran would otherwise face a new round of the United Nations Security Council sanctions that analysts say are already beginning to bite on its economy, he said: "The Iranians know very well what will continue to happen if nothing happens otherwise."
Diplomats said the presence of senior US envoy William Burns at the talks underlined the unity of major powers in the dispute, and stressed that patience was running out with Iran.
The UN has imposed three sets of sanctions on Iran in a stand-off that goes back to the revelation in 2002 by an exiled opposition group of the existence of a uranium enrichment facility and heavy water plant in the country.
Those political and economic sanctions already target the country's banks and include visa bans on officials and measures against companies seen as linked to the nuclear programme.
Iran, the world's fourth-largest oil producer, rejects suspicions that it wants the atom bomb and says its nuclear programme is intended to generate electricity.