Senior European diplomats warned on Tuesday that negotiations to revive nuclear talks could be terminated if Iran moved to produce weapons-grade nuclear fuel.

“It would seriously imperil the process” if Iran did something “as provocative as going to 90% enrichment,” said a senior diplomat from the three Western European nations that are participating in the talks—Britain, France and Germany.

The warning came on the second day of the latest round of talks in Vienna aimed at restoring the 2015 nuclear deal, which lifted most international sanctions on Iran in exchange for tight but temporary restrictions on Iran’s nuclear program.

The European diplomats said it wasn’t yet clear if Iran’s negotiating team, participating in its first round of talks under new hard-line President Ebrahim Raisi, was serious about reaching an agreement. They said Iran needed to negotiate substantively in the next few days.

In April, when the talks on restoring the nuclear deal had started, Iran began producing highly enriched uranium at 60% purity for the first time, partly in response to an explosion at its Natanz nuclear facility that Tehran blamed on Israel. That is a relatively short technical step from producing 90% weapons-grade material. Iran has said in recent months that it could produce 90% enriched uranium, suggesting it could be used for nuclear submarines.

“You cannot enrich to weapons grade and say that you are seeking a return to an agreement whose goal is to ensure the exclusively peaceful nature of Iran’s nuclear program,” a second senior diplomat from the so-called E3 European nations said.

The diplomat said that the Europeans and Americans have warned Iran since April that further advancing its nuclear program would make it harder to restore the 2015 deal, which limits Iran to producing enriched uranium no higher than 3.67% until 2031.

Since the U.S. left the nuclear agreement under former President Donald Trump in May 2018 and imposed sweeping sanctions on Tehran, Iran has gradually expanded its nuclear work and has taken a series of big technical steps this year that could be used for a nuclear-weapons program. Iran says its nuclear program is purely for peaceful, civilian purposes.

Iran has now accumulated a stockpile of enriched uranium more than 11 times the limits set by the 2015 accord and, as of Nov. 6, has produced 17.7 kilograms of 60% enriched uranium, according to the United Nations atomic agency.

Western officials have warned that Iran’s nuclear advances mean that time is running out for restoring the nuclear deal. They have questioned whether Tehran is seeking to run out the clock to enable it to continue its nuclear work.

Iranian officials say they will continue advancing their nuclear program as long as the reimposed U.S. sanctions remain in place.