Iran returned to negotiations over its nuclear program on Monday — meeting for the first time in over five months, with the country’s new hard-line government now in control.

Its chief negotiator emerged from closed doors bullish, as Tehran demands its concerns about continued U.S. sanctions be addressed first after former President Donald Trump withdrew the U.S. from the deal.

But the U.S. and the deal’s European signatories are warning that after months of stalling, Iran is facing its last opportunity to revive the 2015 deal that placed constraints on its nuclear program in exchange for international sanctions relief.

A top European Union diplomat who is coordinating the indirect talks between the U.S. and Iran expressed some guarded optimism afterward — and much urgency.

“There is clearly a will on all the delegations to listen to the Iranian positions brought by the new team, and there is clearly a will of the Iranian delegation to engage in serious work to bring JCPOA back to life,” said Enrique Mora, the senior EU diplomat, using an acronym for the nuclear deal’s formal name — the Joint Comprehensive Plan of Action.

“I feel positive that we can be doing important things for the next weeks to come,” Mora added after delegations from Iran, Russia, China, the United Kingdom, France and Germany met in Vienna, Austria.

Whether or not the U.S. and its European allies are willing to wait weeks is an open question — especially since Ebrahim Raisi, Iran’s new president who is a conservative cleric close to Supreme Leader Ayatollah Ali Khamenei, has delayed the resumption of talks since he won election in June.

“These talks are the last opportunity for the Iranians to come to the table and agree the JCPOA,” British Foreign Secretary Liz Truss said Monday. “We will look at all options if that doesn’t happen.”

Patience is all but out in Israel, whose defense minister warned Monday that Iran is “dashing towards a nuclear weapon.”

Israeli officials shared intelligence with the U.S. and other allies showing that Iran is nearing a nuclear weapon, Israeli Defense Minister Benny Gantz said.

Since Trump’s exit, Iran has increasingly taken steps in violation of the deal, including by enriching more uranium, enriching uranium to higher levels, using more advanced centrifuges and more of them, and enriching uranium metal. The United Nation’s nuclear watchdog — the International Atomic Energy Agency, or IAEA — reported this month that Iran has enriched 39 pounds of uranium to 60%, which is a short technical step from weapons-grade 90%.

Under the nuclear deal, Iran’s enrichment was capped at 3.67% for 15 years.

The State Department declined to comment on reports that Iran may be moving toward 90% enrichment levels, but deputy spokesperson Jalina Porter told reporters that “obviously would be a provocative act, and I’ll just underscore that we’ve made clear that Iran’s continued nuclear escalations are unconstructive and they’re also inconsistent with what’s stated in the goal of returning to a mutual compliance with the JCPOA.”

But ahead of talks resuming, Iran has used sharper language rejecting the idea of “mutual compliance” — increasingly arguing that the U.S. must act first because it was Trump that first exited the deal back in 2018.

“The principle of ‘mutual compliance’ cannot form a proper base for negotiations since it was the U.S. government which unilaterally left the deal,” Iran’s chief negotiator, Ali Bagheri Kani, wrote in an editorial Sunday, calling for a “clear and transparent mechanism to ensure that sanctions will be removed” and U.S. “compensation for the violation of the deal, which includes the removal of all post-JCPOA sanctions.”

The Biden administration has said it will not lift sanctions first, and the idea of compensating Iran for U.S. sanctions is politically toxic in Washington.

It’s unclear if those demands are just Iran posturing before sitting down, or if those are red lines. Out of Monday’s meetings, Bagheri claimed a “considerable achievement” by saying the remaining parties to the deal agreed to address U.S. sanctions first. But that doesn’t mean they agreed those sanctions need to be lifted before Iran’s own non-compliance is addressed. The working-level discussions will address U.S. sanctions on Tuesday and Iran’s nuclear program Wednesday, according to Mora.

The State Department has not yet provided a readout from special envoy for Iran Rob Malley’s meetings in Vienna, where the previous six rounds of talks were held as well.

Beyond Mora’s optimism, Russia’s envoy Mikhail Ulyanov said the talks “started quite successfully” and reached agreement on “further immediate steps,” without specifying what they were.

Any optimism has run face first into dire warnings from Israel, whose Prime Minister Naftali Bennett has vocally opposed the restoration of the nuclear deal.

“Iran deserves no rewards, no bargain deals, and no sanctions relief in return for their brutality. I call upon our allies around the world: Do not give in to Iran’s nuclear blackmail,” Bennett said Monday.

Malley told NPR last week the U.S. and Israel don’t agree on the deal, but do agree on the need to prevent Iran from obtaining a nuclear weapon: “We’re not going to wait and see them get so close,” he said, but the U.S. hopes “that this could be resolved diplomatically, and it should be.”

Amid warnings that Iran could stall by prolonging these talks, Malley added the U.S. will not “sit idly by” if the country moves toward a nuclear bomb.

But the U.S. and European allies have pulled their punches at the IAEA, declining again last week to censure Iran for not just its violations of the deal but its growing obstruction of the IAEA’s work.

Iran has barred inspectors from accessing certain sites, harassed inspectors with invasive security searches and failed to explain still the detected presence of uranium at three undeclared locations, IAEA Director-General Rafael Grossi told the U.N. body last Wednesday.

Grossi visited Tehran last week — his first trip under the Raisi government — but he did not reach a deal to address these issues, he told reporters Wednesday. A previous ad-hoc arrangement with Iran to keep international eyes at its declared nuclear sites is coming apart, he warned. Iran agreed to keep IAEA cameras and other monitoring equipment in place and turn the tapes over to the agency when a deal was reached. That equipment needs servicing to “guarantee continuity of knowledge,” Grossi said, but Iran has blocked IAEA inspectors so far.

“Such a long period of time without us getting access, knowing whether there are operational activities ongoing, is something in itself that would prevent me from continuing to say I have an idea of what’s going on,” he said at a press conference. “We must reach an agreement. We must do it.”