Revived talks aimed at restoring the 2015 Iran nuclear deal ran into difficulties Thursday just days after they restarted, following a report by the International Atomic Energy Agency that Iran has again escalated its enrichment of uranium despite the renewed diplomacy.

In the report, distributed to negotiators late Wednesday, the U.N. body responsible for monitoring Iran’s nuclear program said Iran is enriching uranium to 20 percent purity using advanced centrifuges at its Fordow facility — a place where the original deal prohibited any enrichment at all.

The content of the report was confirmed by IAEA Director General Rafael Grossi, who said increased production at Fordow allows the Iranians “to go much faster and increase [enrichment] volumes significantly.”

The report drew an immediate angry response from Israel. In a phone call with Secretary of State Antony Blinken, Israeli Prime Minister Naftali Bennett urged the United States to call off the talks immediately, accusing Iran of using “nuclear blackmail” as a negotiating technique, according to a statement from his office.

That Iran is stepping up its enrichment of uranium even as it re-engages in diplomacy deepened the sense of pessimism already clouding the talks, which resumed Monday in Vienna after a five-month hiatus brought about by the installation of a new, hard-line government in Tehran.

Speaking to reporters in Stockholm, Blinken said Iran’s “recent moves, recent rhetoric don’t give us a lot of cause for optimism.”

“In the very near future, the next day or so, we’ll be judging whether Iran actually intends now to engage in good faith,” Blinken said. There is still time for Tehran to “reverse course” and engage in meaningful negotiations, he said. “What Iran can’t do is sustain the status quo of building their nuclear program while dragging their feet on talks,” he added. “That will not happen.”