The United States has warned Iran that time is running out to return to a nuclear deal, voicing fears that Tehran’s sensitive atomic activities could advance if talks drag on, Al Jazeera reported.

On the first high-level visit to Paris by President Joe Biden’s administration, Secretary of State Antony Blinken and his French hosts saluted a new spirit of cooperation on Friday after four years of turbulence under Donald Trump.

But the two sides said one key Biden promise—to return to the 2015 accord on the Iranian nuclear program trashed by Trump—was at risk if Tehran does not make concessions during talks that have been going on for months in Vienna.

“There will come a point, yes, where it will be very hard to return back to the standards set by the JCPOA [i.e., the Joint Comprehensive Plan of Action],” Blinken told reporters.

Blinken warned that if Iran “continues to spin ever more sophisticated centrifuges” and steps up uranium enrichment, it will bring nearer the “breakout” time at which it will be dangerously close to the ability to develop a nuclear bomb.

“We have a national interest in trying to put the nuclear problem back in the box that it was in the JCPOA,” Blinken said.