Washington (CNN) -- The United States is tamping down expectations of making any real progress in the latest round of talks this week aimed at getting Iran to stop its nuclear program.
The multilateral talks resume Friday in Istanbul between Iran and six world powers.
Undersecretary of State William Burns is heading the U.S. delegation, joining representatives from Britain, France, Russia, China, Germany and Iran.
State Department Spokesman Mark Toner said Thursday, "We're not expecting any big breakthroughs, but we want to see a constructive process emerge."
The last set of talks in Geneva in December resulted in little progress other than a decision to meet again in Turkey.
Toner did leave open the possibility there could be renewed discussion on a fuel-swap proposal from 2009, under which Iran would exchange with the West some of its low-enriched uranium for processed fuel to use in a small medical research reactor in Tehran.
The proposal by the United States, Russia and France was intended to draw down the amount of enriched uranium Iran would have to use in a possible weapon.
The United States and its allies maintain Iran's uranium enrichment program is designed to build nuclear weapons, despite Iran's claim that its program is for peaceful energy needs.
On the eve of the latest round of talks, a bipartisan group of former high-ranking U.S. officials said time is running out for negotiations. They spoke at a Washington conference sponsored by ExecutiveAction on behalf of its Iranian-American clients, who are opposed to the Tehran regime and support the opposition group Mujahedeen-e-Khalq (MEK).
Michael Mukasey, who served as attorney general under former President George W. Bush, said every attempt by the United States to engage with Iran since its 1979 revolution "has been slightly less successful than the one that preceded it."
Tom Ridge, former head of the Department of Homeland Security, said he doesn't think sanctions will do the trick.
"They won't respond to political pressure; they won't respond to sanctions," Ridge said. "So every legitimate effort, as well-intentioned as they have been, has proven to be unsuccessful."
Both James Jones, Obama's former national security adviser, and former Energy Secretary Bill Richardson support sanctions, though Jones said the latest, more stringent U.N.-supported sanctions will take six months to a year to have their full impact.
One participant, former Sen. Robert Torricelli of New Jersey, suggested a pre-emptive military strike might be the only answer.
Retired Gen. Anthony Zinni said there is a middle ground that would have a greater impact on Iran.
"They fear the opposition more than anything else," Zinni said. "(And) they fear regional cooperation that isolates them."
"Here we have done a lousy job of working within the region to create the kind of security cooperation, the kind of assurances on security that would isolate Iran," said the former commander of forces in the Persian Gulf region. He said military force should be left on the table, but only as a last resort.
Except for Jones, all of the conference participants explicitly favored removing the MEK from the U.S. terrorism list. "There is no reason to be shy about doing more to support the Iranian opposition," said former State Department official Mitchell Reiss. "A good step would be delisting the MEK."
The MEK, which opposes the Iran regime, was placed on the terrorism list in 1997 by the Clinton administration in an effort to reach out to what was believed at the time to be a more liberal Iranian leadership.
Mukasey underscored MEK's assistance to the United States and its ability to undermine the regime.
"It is certainly helpful that MEK remain a bone in the throat of Iran. ... It has provided valuable intelligence and information on the Iranian nuclear program to the U.S.," Mukasey said. "It would be fair to say the U.S. would not know a great deal about Iran's nuclear program without the information obtained by MEK."
In an interview last week with CNN's Jill Dougherty, Secretary of State Hillary Clinton bristled at the suggestion there has been no progress in persuading Iran to end its nuclear program.
"We have made progress," she said. "In the last two years, we moved from a policy of condemning and standing alone and seeing nothing happen to rallying the international community to impose very tough sanctions, which are making a difference."
Clinton said the path to the desired changes from Iran is "very long and challenging."