U.S. envoy for Iran Robert Malley was on Friday to meet diplomats from three European powers over the Iranian nuclear crisis as Vienna-based talks to salvage a 2015 deal remain suspended.

The agreement between Iran and world powers to find a long-term solution to the now two-decade-old crisis over its controversial nuclear program has been moribund since former U.S. President Donald Trump walked out of the deal in May 2018.

His successor Joe Biden has said he is ready to re-enter the agreement, so long as Iran meets key preconditions including full compliance with the deal whose terms it has repeatedly violated by ramping up nuclear activities since Trump walked out.

But the Vienna-based talks through intermediaries made little headway, before being interrupted by the election of Ebrahim Raisi as Iran's president and suspended for the last four months. Malley's trip to Paris for the meeting with the so-called E3 of Britain, France and Germany comes after he visited the Gulf for talks with allies Qatar, Saudi Arabia and the United Arab Emirates (UAE) who are all deeply concerned by Iran's nuclear program.

"Following consultations with partners in the region, Special Envoy for Iran Robert Malley will meet with his E3 counterparts in Paris on Friday," a U.S. State Department spokesperson said, without giving further details.

The French Foreign Ministry said in a statement that the talks were coming at a "critical time" when France and other world powers were still prepared to return to the Vienna talks on bringing the U.S. back into the deal.

"In the meantime, it is urgent and essential that Iran ends violations of unprecedented gravity" of the nuclear accord, the ministry said, urging Iran to also resume full cooperation with the U.N. atomic agency "without delay."

Western powers, Israel and pro-Washington Arabian Peninsula states fear that Iran intends to develop an atomic bomb. Tehran denies this, insisting it only seeks to produce energy for its population. The nuclear deal promised Iran step-by-step sanctions relief in exchange for restrictions on its atomic work which would be under the strict supervision of the U.N. atomic agency.

U.N. nuclear watchdog chief Rafael Grossi expressed concern Tuesday he was still waiting for a "high level" discussion with Iranian officials in Tehran, after negotiating last month a new compromise on monitoring Iran's nuclear program to help restart the talks in Vienna.