U.S. and Iranian officials clashed on Friday over what sanctions the United States should lift to resume compliance with the 2015 nuclear deal. Talks are likely to continue for weeks to see whether there will be more progress.

The two nations laid out tough stances as indirect talks in Vienna on how to bring both back into full compliance with the agreement wound up for the week, with some delegates citing progress.

The talks, with European Union officials as intermediaries, aim to restore the bargain at the core of the agreement - restrictions on Iran's nuclear activities in exchange for the lifting of U.S. and other international sanctions.

Former U.S. President Donald Trump withdrew from the deal, which lifted economic sanctions on Iran in return for curbs to its nuclear program. He reimposed U.S. sanctions, prompting Iran in turn to violate the accord's atomic limits.

"All Trump sanctions were anti-JCPOA & must be removed—w/o distinction between arbitrary designations," Iranian Foreign Minister Javad Zarif said on Twitter.

The U.S. says it is prepared to lift "sanctions that are inconsistent with the JCPOA." However, it has declined to give any details, that appears to exclude sanctions formally unrelated to nuclear issues covered by the deal.

Read more:

Vienna talks: U.S. sanctions to be lifted and a 'new chapter' for Iran

Chinese envoy urges U.S. to lift all illegal sanctions against Iran as nuclear talks resume

The remaining parties to the accord - Iran, Britain, China, France, Germany and Russia - met again on Friday after talks formally began on Tuesday and they agreed to keep going, Russian and Chinese envoys said.

"The #JCPOA participants took stock of the work done by experts over the last three days and noted with satisfaction the initial progress made," Mikhail Ulyanov, Russia's envoy to the United Nations' International Atomic Energy Agency (IAEA), said on Twitter after the meeting formally known as the Joint Commission.

"The Commission will reconvene next week in order to maintain the positive momentum."

The remaining parties have formed two expert-level working groups whose job is to draw up lists of sanctions that the United States will lift and of nuclear restrictions Iran will implement. Their work continues between Joint Commission meetings.

"All parties have narrowed down their differences and we do see the momentum for gradually evolving consensus," said Wang Qun, China's ambassador to the IAEA.

Iran's foreign ministry said in a statement that diplomats would meet again on Wednesday in Vienna. Talks are expected to drag on for weeks in consideration of the technical complexity of the nuclear aspects and legal intricacies of sanctions lifting.

Khamenei, who has the last say on all state matters, has opposed any gradual easing of sanctions.

(With input from Reuters)