Efforts to revive Iran's 2015 nuclear deal cannot succeed unless Washington lifts "major" sanctions, Iranian President Ebrahim Raisi said on Monday, as sources said indirect talks between Iran and the United States were reaching a final stage.

Reuters reported last week that a U.S.-Iranian deal is taking shape in Vienna after months of indirect talks to revive the nuclear pact abandoned in 2018 by then-U.S. President Donald Trump, who also reimposed extensive sanctions on Iran.

"The United States must prove its will to lift major sanctions," Raisi said at a joint news conference with Qatari Emir Sheikh Tamim bin Hamad al-Thani in Doha.

"To reach an agreement, guarantees are necessary for negotiations and nuclear issues."

Iran had demanded legal assurances that the United States will not exit the deal again but Washington says it is impossible for U.S. President Joe Biden to provide them. A senior Iranian official told Reuters that Tehran had shown flexibility in response 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.

"Aggression is bound to fail. Resistance has brought results and none of the regional issues have a military solution," Raisi, a hardliner, said in Doha.

He was more cautious than Iranian foreign ministry spokesman Saeed Khatibzadeh, who said earlier on Monday that the Vienna negotiations had made "significant progress".

Khatibzadeh added, however, that "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 that some minor technical issues were being discussed and that a deal was expected before end of the week.

Khatibzadeh said the talks were being handled by Iran's top security body, the Supreme National Security Council, which reports directly to Supreme Leader Ayatollah Ali Khamenei.

The 2015 deal between Iran and world powers limited Tehran's enrichment of uranium to make it harder for it to develop material for nuclear weapons, if it chose to, in return for a lifting of international sanctions against Tehran.

Since 2019, Tehran has gone well beyond the deal's limits, rebuilding stockpiles of enriched uranium, refining it to higher fissile purity and installing advanced centrifuges to speed up output.

Iran's foreign minister said on Saturday that Tehran is ready to swap prisoners with the United States, and that talks to revive the nuclear deal could succeed "at the earliest possible time" if Washington made the necessary political decisions.

Speaking from Doha on the sidelines of a gas conference, Iranian Oil Minister Javad Owji called the sanctions, which have squeezed Iran's oil exports, a violation of international law and a threat to global energy security, the semi-official Tasnim news agency reported.

Israel, Iran's arch-foe and widely believed to have its own nuclear arsenal, has been closely watching the Vienna talks. It is pressing Washington about the terms of an emerging Iranian nuclear deal, Israeli officials said on Monday, raising the prospect of a bilateral day-after agreement with Washington to address their worries.

While not a party to the talks, Israel has conferred with the U.S. administration in hope of wielding more clout over any revival of the deal with Tehran reached over its objections.