Biden told the Munich Security Conference that the United States would work closely with allies in dealing with Iran after his predecessor Donald Trump took an aggressive unilateral approach.

"The threat of nuclear proliferation also continues to require careful diplomacy and cooperation among us," Biden told fellow leaders via teleconference.

"That's why we have said we're prepared to reengage in negotiations with the P5+1 on Iran's nuclear program," he said, referring to the five permanent members of the UN Security Council and Germany.

"We must also address Iran's destabilizing activities across the Middle East, and we're going to work in close cooperation with our European and other partners as we proceed."

- Opening moves toward talks -

Biden's remarks came after his administration agreed to return to nuclear talks with Iran led by European allies, after Trump withdrew from the 2015 agreement which had put a hold Tehran's nuclear program.

After Trump pulled out in 2018 and aggressively placed sanctions on many Iranian individuals and organizations to punish the country, the Islamic republic resumed enrichment and development activities in alleged violation of the pact.

On Thursday State Department spokesman Ned Price said Washington had accepted an invitation by the European Union political director Enrique Mora for an "informal" meeting involving Iran.

Price said the meeting would "discuss a diplomatic way forward."

At the same time, in an opening gesture, the Biden administration dropped a push for more sanctions at the UN crafted by Trump, and removed restrictions on Iranian diplomats accredited to the United Nations in New York.

Tehran however responded by renewing its demand for US to lift all sanctions imposed by Trump, maintaining a threat to restrict access to UN nuclear inspectors.

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."

Earlier Thursday US Secretary of State Antony Blinken warned jointly with European powers that restricting the inspectors would be a "dangerous" move.

- Reversing Trump steps -

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

"Gestures are fine. But to revive P5+1, US must Act: LIFT sanctions. We WILL respond," the spokesman tweeted.

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.

Biden was not specific about what he meant by Iran's destabilizing activities.

But his comments came in the wake of a missile attack on a military base housing US and coalition forces in northern Iraq that killed a foreign contractor working for the US military and injured 14, including a US soldier and contract workers.

Washington has not laid blamed for the attack. But the missiles appeared identical to those used by Iran-backed Iraqi armed groups in attacks in Baghdad.

After the US opening Thursday, White House spokeswoman Jen Psaki signaled that the ball was now in Iran's court.

Biden reiterated "an openness to having a diplomatic conversation," using the same channels of the past with the P5+1 partners, said Psaki.

"We don't anticipate taking additional steps," she said.