Foreign Minister Mohammad Javad Zarif tweeted that Iran would "immediately reverse" its retaliatory measures if the US lifts "all sanctions imposed, re-imposed or re-labelled by Trump".

The Biden administration on Thursday offered talks with Iran led by European allies and reversed two largely symbolic steps against Tehran imposed by Trump, as it sought to salvage a nuclear deal on the brink of collapse.

Ahead of a Sunday deadline set by Iran for it to restrict some access to UN nuclear inspectors unless sanctions are lifted, new US Secretary of State Antony Blinken warned jointly with European powers that the move would be "dangerous".

Hours after Blinken's videoconference with his French, British and German counterparts, the European Union political director, Enrique Mora, proposed via Twitter an "informal meeting" involving Iran -- and the US accepted.

"The United States would accept an invitation from the European Union High Representative to attend a meeting of the P5+1 and Iran to discuss a diplomatic way forward," said State Department spokesman Ned Price.

The P5 -- UN Security Council powers Britain, China, France, Russia and the US -- plus Germany sealed the 2015 deal brokered by then president Barack Obama under which Iran drastically scaled back its nuclear programme in exchange for promises of economic relief.

Trump withdrew from the deal in 2018 and re-imposed sweeping sanctions, aiming to bring Iran to its knees.

- Reversing Trump steps -

Iran's foreign ministry spokesman Saeed Khatibzadeh said there is currently no such thing as P5+1 "because of US withdrawal" from the agreement.

"Trump left the room and tried to blow it up," the spokesman tweeted. "Gestures are fine. But to revive P5+1, US must Act: LIFT sanctions. We WILL respond. Here is the key sequence: #CommitActMeet."

Zarif did not explicitly address the Biden administration's offer of talks. Iran has demanded an end to Trump's sanctions before reversing protest measures it began almost a year after the US withdrawal.

A senior US official said the Biden administration was showing good faith and saw a meeting as the start of a "prolonged path" to restoring and building on the nuclear accord.

If Iran declines to meet, "I think it would be... unfortunate," the official said on condition of anonymity.

Britain and Germany swiftly welcomed the proposed talks.

Chancellor Angela Merkel's spokesman Steffen Seibert said it was "crucial to seize this diplomatic opportunity, and therefore it is crucial not to take unilateral measures that go against it".

Russia said the US "refusal to call for sanctions is a good thing", but what was needed was the full return of the 2015 deal.

Biden has insisted he will not lift Trump's sanctions until Iran returns to compliance -- but the administration Thursday undid two symbolic steps by its predecessor.

In a letter to the United Nations, the US said it no longer believed that the world body had "snapped back" sanctions on Iran.

Blinken's predecessor Mike Pompeo last year argued the United States was still a "participant" in the Security Council resolution that blessed the nuclear deal -- despite withdrawing later -- and therefore could reimpose sanctions.

The argument had been dismissed by the United Nations and close US allies at the time.

Zarif said Iran agreed with the Biden administration's decision.

Washington also reversed draconian curbs on Iranian diplomats in New York, who were barred from all but a few blocks around the United Nations and their mission.

- Warning over inspections -

Under a bill adopted by its conservative-dominated parliament in December, Iran will restrict some inspections by the International Atomic Energy Agency if US sanctions are not lifted.

IAEA chief Rafael Grossi is to travel to Tehran on Saturday for talks, a day before the February 21 deadline, with restrictions set to begin two days later.

The United States and Iran have had no diplomatic relations for four decades but they began frequent contact to negotiate the 2015 nuclear deal.

The nuclear accord was adamantly opposed by Iran's regional rivals Israel and Saudi Arabia, which both enjoyed tight partnerships with Trump.

While Iran's policy is ultimately determined by supreme leader Ayatollah Ali Khamenei, elections in June add another time pressure factor, with President Hassan Rouhani, a key advocate of nuclear diplomacy, set to step down.