As indirect nuclear talks between Iran and the United States entered it fourth day in Vienna, the two countries’ officials clashed over what sanctions should be lifted to allow both parties to return to compliance with the 2015 nuclear deal.

Both countries stuck to their position, with Iran demanding the removal of all sanctions imposed by the former US administration, while the new team in the White House says only sanctions that are not compatible with the Joint Comprehensive Plan of Action (JCPOA) or Iran’s nuclear agreement with world powers.

The talks, in which European Union officials are shuttling between the remaining parties to the deal and the United States, aim to restore the bargain at the core of the agreement - restrictions on Iran's nuclear activities in exchange for the lifting of US and other international sanctions.

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

The United States says it is prepared to lift "sanctions that are inconsistent with the JCPOA". While it has declined to elaborate, that appears to exclude sanctions formally unrelated to nuclear issues covered by the deal.

A senior US State Department official told reporters the United States had seen some signs of Iranian seriousness about returning to the nuclear pact but "certainly not enough."

"If Iran sticks to the position that every sanction that has been imposed since 2017 has to be lifted or there will be no deal, then we are heading towards an impasse," the senior US official told reporters on a conference call.

Whether the statements are opening gambits or more firm positions remains to be seen. European officials said Iran was bargaining hard at the outset.

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 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.

Some diplomats hope agreement can be reached before Iran's June 18 presidential election or else talks risk being pushed back until later in the year.

"Iran is the pace car for progress. If Tehran decides to push forward swiftly before the June presidential elections, the US will almost certainly be receptive," Henry Rome, an analyst with the Eurasia Group research firm, said in a note.

Reporting by Reuters