Diplomats confirmed the new development to the Associated Press on Saturday.
The annex, one of five meant to accompany the agreement, outlines which U.S. and international sanctions will be lifted and how quickly.
Diplomats said senior officials of the seven-nation talks, which include U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif, still had to sign off on the package.
Still, the word of significant progress indicated the sides were moving closer to a comprehensive accord that would set a decade of restrictions on Tehran's nuclear program in exchange for tens of billions of dollars' in economic benefits for the Iranians.
Officials had described sanctions relief as one of the thorniest disagreements between Iran and the United States, which has led the international pressure campaign against Iran's economy.
The U.S. and much of the world fears Iran's enrichment of uranium and other activity could be designed to make nuclear weapons; Iran says its program is meant only to generate power and for other peaceful purposes.
The diplomats, who weren't authorized to speak publicly on this past week's confidential negotiations in Vienna, said the sanctions annex was completed this week by experts from Iran and the six world powers it is negotiating with: the United States, Britain, China, France, Germany and Russia. They did not provide details of the agreement.
A senior U.S. official did not dispute the diplomats' account but said work remained to be done before the issue could be described as finalized.
Negotiators are striving to wrap up the deal by July 7.
Along with inspection guidelines and rules governing Iran's research and development of advanced nuclear technology, the sanctions annex of the agreement had been among the toughest issues remaining to be resolved.
Iranian officials, including Supreme Leader Ayatollah Ali Khamenei, have made repeated demands for economic penalties to be lifted shortly after a deal is reached. Washington and its partners have said they'd take action after Iran verifiably complies with restrictions on enrichment and other elements of the nuclear program.
Much of the negotiation on the matter has concerned sequencing, so that both sides can legitimately claim to have gotten their way.
Several other matters related to sanctions also had posed problems.
The Obama administration cannot move too quickly to remove economic penalties because of Congress, which will have a 30-day review period for any agreement during which no sanctions can be waived.
American officials also had been struggling to separate the "nuclear-related" sanctions it is prepared to suspend from those it wishes to keep, including measures designed to counteract Iranian ballistic missile efforts, human rights violations and support for U.S.-designated terrorist organizations.
And to keep pressure on Iran, world powers had been hoping to finalize a system for snapping suspended sanctions back into force if Iran cheats on the accord.
Russia has traditionally opposed any plan that would see them lose their U.N. veto power and a senior Russian negotiator said only this week that his government rejected any automatic "snapback" of sanctions.