American and Iranian negotiators raced to fill out a framework for rolling back Iran's nuclear program and punitive U.S. economic sanctions, hoping for enough progress to call in other world powers for the finishing touches on an agreement next week.

U.S. Energy Secretary Ernest Moniz and Iranian atomic energy chief Ali Akbar Salehi met again Wednesday in the Swiss city of Lausanne to discuss the technical obstacles to a deal. Secretary of State John Kerry and Foreign Minister Mohammad Javad Zarif were to join the talks later.

Negotiations are expected to continue until Friday. And although neither side is promising a breakthrough over the next three days, each is hoping to resolve as many lingering issues as possible, from the speed of a U.S. sanctions drawdown to the level of inspections on Iranian nuclear sites.

A sign of an impending deal would be the discussions wrapping up with an announcement of more talks next week and the involvement of America's negotiating partners: Britain, China, France, Germany and Russia. Up to now, Washington and Tehran have mainly negotiated between each other, but all seven countries would have to sign off on an accord.

The governments have set for themselves a deadline on a preliminary deal by the end of March, with a full agreement by July.

Salehi, an MIT-educated physicist and former Iranian foreign minister, suggested Tuesday a deal was close, saying one "final item" remained contentious. He didn't specify, but said that matter's resolution would mean "on technical issues, things are clear on both sides."

"As a whole, I am optimistic," he told reporters.

The United States was less upbeat. "There's no doubt they have made substantial progress over the past year," White House spokesman Josh Earnest said. But he declared the chances of an agreement "at best 50-50."

Washington wants to stretch the time Iran would need to make a bomb from a few months to a year. The deal taking shape would limit Iran's uranium enrichment and other nuclear activity for at least a decade, with the restrictions slowly lifted over several years.

Congressional Republicans have threatened to upend the diplomacy, claiming any deal would be ineffective. And Prime Minister Benjamin Netanyahu, whose party scored a resounding victory in Israel's election Tuesday, is also an opponent.