"We are close to a possible deal, but we're not there yet," said State Department spokesman Ned Price. "We do think the remaining issues can be bridged."

Speaking to reporters, Price declined to confirm Tehran's claim that there were just two final issues to be sorted out before agreeing to restore the six-party Joint Comprehensive Plan of Action (JCPOA) which aimed to prevent Tehran from developing nuclear weapons.

The outstanding issues, according to sources close to the talks, are Iran's demands for "economic guarantees" in case a future US administration changes its stance and abrogates the agreement, as president Donald Trump did in 2018; and that Washington remove its official terror group designation on Iran's powerful Revolutionary Guards.

Price said the issues are surmountable but that the 11-month-old talks "are at a very delicate stage."

"There is little time remaining given the nuclear advancements that Tehran has made" toward developing nuclear weapons that would undermine any agreement," he said.

"This is an issue that needs to be worked urgently," he added.

Even if a deal is near, the State Department's top negotiator in the Vienna talks, Rob Malley, remains in Washington, suggesting that nothing would be signed in the coming days.

Also possibly delaying a deal is the Iranian Nowruz New Year celebration which takes place on Sunday.