Any easing by Washington of sanctions on Iran in exchange for Tehran taking steps to scale back its nuclear program is likely to be fleeting and symbolic, with any moves for bigger concessions likely to be blocked by Congress.
At talks in Geneva on the nuclear program with six world powers on Tuesday, Iran's negotiators presented a proposal on defusing a decade-old standoff. But both sides said it was too early to talk of a breakthrough.
Even if Iran promises to take serious steps, it is unlikely to satisfy key members of the US Congress, which generally takes a harder line on Iran than President Barack Obama's administration.
Lawmakers including Robert Menendez, the chairman of the Senate Foreign Relations Committee, have signaled they want Tehran to stop even low-level enrichment of uranium used in generating power before they would take steps to wind down existing sanctions, or even agree not to put through tougher ones.
"Sanctions relief is easier said than done," said Ali Vaez, an Iran analyst at the International Crisis Group, an organization that seeks to prevent and resolve conflict.
"Without a fundamental reorientation of Iran's approach, a significant relaxation in sanctions is not in the cards."
The sanctions Washington would likely wind down first are morsels such as easing restrictions on medical supplies, travel and the sale of spare airplane parts and service for US-built aircraft in Iran.
"Many of these are low impact, they are not going to turn Iran's economy around," said Greg Theilmann, a fellow at the Arms Control Association, and a former top intelligence official at the State Department.
"But it's important to at least know there are a number of things that can be done to show US bona fides if the Iranians show that they are willing to take significant steps in the direction of what the US and other parties say they have to do," he said.
Deadly crashes of aging aircraft owned by Iranian airlines have become common and Iranian officials say their inability to buy new Western-built planes and parts have led to more than 1,700 deaths since those sanctions began in 1995.
Allowing access to US aircraft parts could help improve relations between the two countries as negotiators work on more difficult sanctions that are crippling Iran's economy.
Property, bank accounts
A congressional aide suggested sanctions on Iran's property and accounts in the United States - estimated to be worth billions of dollars - on oil transactions, and access to the banking system could be relaxed in stages over many months, if Iran were to make verified major concessions.
"You could restore access for a defined number of Iranian banks, as opposed to all of them, put limits on the size of the transactions that could be processed," the aide said. "You can calibrate it all kinds of different ways."
With Iran indicating a new willingness to engage with its adversaries, the chief US negotiator at the Geneva talks on Tuesday held a rare bilateral discussion with Tehran's delegates, described by a senior US official as "useful."
Hopes for a deal on Iran's nuclear ambitions rose when Obama spoke by telephone last month with Iran's new president, Hassan Rouhani, a self-described moderate. It was the highest level contact between the two countries since the 1979 Islamic Revolution that led to students taking Americans hostage in the US Embassy in Tehran.
The Democratic president could also exercise temporary waivers of sanctions on oil sales that have slashed Iran's crude exports by more than 1 million barrels per day since 2011, depriving the country of billions of dollars worth of sales per month.
With a waiver, Obama could suspend sanctions for 120 days before they kick in again.
"You have to ask yourself what's the way to offer sanctions relief that's reversible if Iran reneges on its parts of any bargain," said Michael Singh, a managing director at The Washington Institute, who was a senior director for Middle Eastern affairs under former President George W. Bush.
But even waivers could risk a backlash from Congress, at a time when Obama already has his hands full with fiscal fights with Republicans that have brought the United States to the brink of a debt default.
Pressuring Iran is one of the rare issues both Democrats and Republicans agree upon and powerful pro-Israel lobbying groups hold great sway among lawmakers from both parties who worry Obama will give up too much in the current nuclear talks.
Temporary US waivers on major sanctions on oil and banking access are unlikely to immediately prompt meaningful and irreversible Iranian concessions. But if sanctions are relaxed in stages over time, it could lead to progress on a longer-term basis.
Washington and its allies believe Tehran is developing the ability to make a nuclear weapon; Tehran says the program is for generating power and medical devices.
Complicating the matter, not all US sanctions are tied to the nuclear issue. Some also deal with Iran's human rights record and sponsorship of militant groups such as Hezbollah.