An agreement between Iran, the US and other world powers could be finalised in Vienna within the next couple of days, according to officials.
Negotiators involved in reviving the Iran nuclear deal, however, are still discussing substantial final demands from Iran, including the scope of sanctions relief, said the officials, who spoke to The Wall Street Journal.
A draft of the deal has focused on the unfreezing of Iranian funds and the release of western prisoners but does not include waivers on oil sanctions, diplomats said.
The talks began early in President Joe Biden's term before taking a pause to allow for a new Iranian president to take office. They have been moving ahead at a rapid rate since November.
The goal of the talks, taking place at Vienna's Hotel Palais Coburg, has been to find a path to restore the 2015 nuclear deal, which the US pulled out of.
The talks involve all the original signatories of the JCPOA — Britain, China, France, German, Russia, the US and the EU.
Other parties in Vienna, including France, China and Iran, have, over the course of the past week, said a deal could be on the horizon.
Iranian foreign ministry spokesman Saeed Khatibzadeh said on Monday the Vienna negotiations had made "significant progress".
Mr Khatibzadeh said, however, "nothing is agreed until everything is agreed", telling a weekly press briefing: "The remaining issues are the hardest".
Two sources close to the Vienna talks told Reuters some minor technical issues were being discussed and a deal was expected before the end of the week.
Iran has reportedly demanded legal assurances that the US will not exit the deal again but Washington says it is impossible for President Biden to provide them. A senior Iranian official told Reuters Tehran had shown flexibility by agreeing to "inherent guarantees".
A draft text of the agreement also alluded to other issues, including unfreezing billions of dollars in Iranian funds in South Korean banks and the release of western prisoners held in Iran.