ISTANBUL – The collapse of another attempt at international outreach to Iran on Saturday has left world powers with few options except to wait — and hope that the bite of sanctions will persuade Tehran to reconsider its refusal to stop activities that could be harnessed to make nuclear weapons.

But their patience could be tested. While the U.S. and others say that Iran already is suffering from the wide range of financial and trade sanctions, travel bans and other penalties imposed by the U.N., the U.S., the EU and others, the Islamic Republic shows no sign of bending.

Uranium enrichment lies at the heart of the dispute.

Low-enriched uranium — at around 3.5 percent — can be used to fuel a reactor to generate electricity, which Iran says is the intention of its program. But if uranium is further enriched to around 90 percent purity, it can be used to develop a nuclear warhead.

Iran came to the Istanbul talks with six world powers Friday declaring it would not even consider freezing uranium enrichment — and left the negotiations Saturday repeating the same mantra. Throughout two days of hectic meetings, it stubbornly pushed demands it must have known were unacceptable to the six — a lifting of sanctions and acceptance of its enrichment program before any further discussion of its nuclear activities.

"Both these preconditions are not the way to proceed," EU foreign policy chief Catherine Ashton declared — and added no new talks were planned.

Publicly, the U.S. and others nations concerned that Iran could turn its enrichment program toward making fissile warhead material say that troubles with enrichment have slowed that activity and left more time to persuade Iran to heed international concerns than thought just a year ago.

Israeli officials now talk of a three-year window — until 2014 — before Iran can make a bomb. That compares with projections of 2011 just three years ago.

U.S. Secretary of State Hillary Rodham Clinton told NBC's "Today" show earlier this week that the new Israeli estimates are "very significant." The delay, she said, "gives us more of a breathing space to try to work to prevent them from obtaining a nuclear weapon."

Two outside forces could account for any Iranian problems in enriching uranium — the increasing weight of U.N. and other sanctions meant to choke off raw materials needed to make and maintain the program, and the apparent havoc caused by the mysterious Stuxnet computer malware, which experts think was created by Israel or the U.S.

"Sanctions have had an impact. There are signs that Iran's nuclear program has slowed, so I think there is time and space for diplomacy," a senior U.S. administration official said after the Istanbul talks collapsed. He asked for anonymity in exchange for discussing the delicate issue.

While there is no talk for now of U.N Security Council new sanctions past a fourth set in June that target Iran's Revolutionary Guard, ballistic missiles and nuclear-related investments, there have been significant Western efforts to enforce present penalties.

Senior U.S. officials have been touring China, Japan, South Korea and the pro-western Arab nations to demand compliance with the U.N sanctions and the European Union, Canada, Australia and others have followed Washington's example in imposing their own restrictions on trade, financial transactions and other relations with Iran.

A diplomat from a delegations that met with Iran in Istanbul told the AP Saturday that EU outsiders Switzerland and Norway would formally adopt the EU sanctions next week. He asked for anonymity because he was not authorized to discuss the matter.