Iranian Deputy Foreign Minister Abbas Araqchi arrives at the venue of Iran nuclear deal talks in Vienna, Austria, April 6, 2021. (Photo: Xinhua)

The United States said Tuesday it had seen possible progress in talks with Iran but joined European negotiators in pressing for urgency in rolling back Tehran's nuclear program.

Negotiations resumed Monday in Vienna in a fresh push to make headway on reviving a landmark 2015 agreement that curtailed Iran's nuclear activities in return for sanctions relief.

"There may have been some modest progress," State Department spokesman Ned Price told reporters in Washington.

"But it is in some ways too soon to say how substantive that progress may have been. At a minimum any progress, we believe, is falling short of Iran's accelerating nuclear steps and is far too slow."

Former president Donald Trump in 2018 withdrew from the nuclear accord and imposed a slew of punishing sanctions, including a unilateral US ban on Iran selling its key export of oil. President Joe Biden supports a return to the agreement but Iran has kept taking steps away from compliance as it presses for sanctions relief.

The Vienna talks began after Biden's election but stopped in June as Iran elected a new ultraconservative government. They resumed in late November with Iran agreeing to keep talking after a brief break.

"This negotiation is urgent," negotiators from Britain, France and Germany said.

"We are clear that we are nearing the point where Iran's escalation of its nuclear program will have completely hollowed out the JCPOA," the so-called E3 powers said, referring to the deal's official name by its acronym.

"That means we have weeks, not months, to conclude a deal before the JCPOA's core non-proliferation benefits are lost."

Israel, Iran's archenemy, has warned of military options if the Islamic republic's program advances and is suspected in a shadowy campaign that has included the assassination of Tehran's top nuclear scientist. The Biden administration has also warned of a return to pressure if talks fail and Iran pursues its nuclear work.

Iran was in compliance with the 2015 deal before Trump's withdrawal but has since taken key steps including stepping up its enrichment of uranium, although it denies that it wants to acquire a nuclear arsenal.

On Saturday, Atomic Energy Organization of Iran director Mohammad Eslami said Tehran had no plans to enrich uranium beyond 60 percent, even if the Vienna talks fail.

Eslami said the enrichment levels were related to the needs of the country, in remarks published by the Russian news agency RIA Novosti.

In response, E3 negotiators said Tuesday that 60 percent enrichment was still "unprecedented for a state without nuclear weapons." Military-grade levels are around 90 percent.

"Its increasing 60 percent stockpile is bringing Iran significantly closer to having fissile material, which could be used for nuclear  weapons," they said.