Dalton said the negotiators may have to try for a more limited first stage accord if their current effort fails.

Already, these talks are intended only to reach a preliminary agreement on confidence building steps. Then there would be a six-month negotiating period to reach a full accord and potentially end all nuclear-related economic sanctions, which have devastated Iran's economy.

No one is saying exactly what the gaps are, but issues center on Iran's extensive capability to enrich uranium to near weapons grade, and its demands for sanctions relief in return for freezing, and perhaps rolling back, parts of its nuclear operation. Iran says it does not want to build a nuclear bomb, but the Security Council wants proof, through the elimination of the highly enriched stockpile and verifiable restrictions for the future.

Iran wants recognition of what it sees as its right to enrich uranium. And it apparently wants more immediate sanctions relief than the U.N. team is willing to give.

Both sides are under pressure to move quickly.

In Iran, hardliners are eager to declare the failure of the relatively moderate government that was elected in June with a mandate to seek an end to the sanctions.

In the United States, a key member of the contact group and some members of Congress want to add sanctions to force Iran to make more concessions, a move the Obama administration says could kill any chance of a peaceful resolution to the dispute.