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By Andrea Mitchell

Experts helping to negotiate the Iran nuclear talks have reached a tentative agreement on how and when to lift U.S. and European Union sanctions against Iran — but the nations' foreign ministers must still accept the deal, NBC News has learned.

While officials say they have narrowed the gaps on all issues, some sticking points remain ahead of Tuesday's deadline.

One of the toughest sanctions issues still unresolved: How the separate United Nations sanctions would be "snapped back on" if Iran were found to have violated any part of the final agreement with the world powers.

Related: Iran: 'Good' Nuke Deal Could Lead to Cooperation in Fight Against Extremists

The U.S., along with Britain, China, France, Germany and Russia, are seeking to work out a deal to keep Iran from developing nuclear weapons. In exchange, economic sanctions levied against Iran — and in place for decades — would be lifted.

Related: Kerry: Iran Negotiators Making Progress on Nuclear Deal

After months of talks, senior U.S. officials have frequently cautioned that they can get close to a deal, but still become stuck on a few final obstacles.

Among the unresolved issues include giving access for inspectors to nuclear sites and suspicious sites on or near military bases, as well as how much research and development Iran can do at different facilities 10 or 15 years down the line.

The foreign ministers are expected to rejoin Secretary of State John Kerry and Foreign Minister Mohammad Javad Zarif in Vienna on Sunday, according to the current plan. The earliest a deal could be announced would be Monday.

Erik Ortiz contributed.