Major powers gave Iran two weeks to answer calls to rein in its nuclear programme on Saturday or face tougher sanctions after talks ended in stalemate despite unprecedented U.S. participation.
A U.S. 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 after some six hours of talks in Geneva with Iran's Saeed Jalili and envoys from the United States, Russia, China, France, Germany and Britain - the so-called sextet of world powers.
Solana said he hoped for a clear answer from Tehran in around 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 U.N. Security Council sanctions that analysts say are already beginning to bite on its economy, he told a news conference:
"The Iranians know very well what will continue to happen if nothing happens otherwise."
Diplomats said the presence of senior U.S. envoy William Burns at the talks underlined the unity of major powers in the dispute, and stressed that patience was running out with Iran.
"There is nothing more to talk about. The Iranians are running the risk of foreclosing their options," said one diplomat in Gevena, warning they risked "going down the path which means further measures in the EU and the U.N."
Solana said he hoped for more contacts with Iran "telephonically or physically", but officials made clear that any subsequent contacts would be at a lower level than Saturday's talks.
