While President Joe Biden works to address the crisis over Russia menacing Ukraine, there is another critical one looming, with a senior State Department official telling reporters the “end game” is just weeks away.

Iran nuclear talks are scheduled to resume this week for their ninth round — with the U.S. and Iran still negotiating indirectly about both countries returning to the Obama-era nuclear deal that is in tatters.

This could be the final round before a deal is reached or the U.S. and its European allies call it quits — because after 10 months of negotiating with two different Iranian governments, the country’s nuclear program is advancing to the point of no return, the U.S. says.

“This can’t go on forever because of Iran’s nuclear advances. This is not a prediction. It’s not a threat. It’s not an artificial deadline. It’s just a requirement… Given the pace of Iran’s advances, its nuclear advances, we only have a handful of weeks left to get a deal — after which point it will unfortunately be no longer possible to return to the JCPOA and to recapture the nonproliferation benefits that the deal provided for us,” said the senior State Department official, who briefed reporters on condition of anonymity to discuss the talks using an acronym for the deal’s formal name, the Joint Comprehensive Plan of Action.

Some critics have said the U.S. should have called it quits long ago, with Iran enriching uranium up to 60% and enriching uranium metal, spinning more advanced centrifuges and more of them, and obstructing access for the U.N.’s nuclear watchdog, the International Atomic Energy Agency. Nuclear weapons-grade uranium is enriched at 90%, while the nuclear deal capped Iran’s enrichment at 3.67% for 15 years.

With those steps, Iran is now a matter of weeks away from having enough fissile material to build a nuclear bomb, the official said. Secretary of State Antony Blinken and others have warned for a couple of weeks now that Iran is just a “few weeks” away from that critical threshold — although the senior State Department official said it would take some additional time to actually build a nuclear bomb, declining to provide a timeline for that.

“Do the math. There are many fewer weeks left now than there were when we first said it,” they added.

That puts the pressure on this new round of talks to reach a conclusion before time runs out. The Iranian delegation returned to Tehran after the eighth round broke up last Friday, just as chief U.S. negotiator Rob Malley returned to Washington.

Ahead of talks resuming, the senior State Department official said Biden remains ready to make that decision and return to compliance by lifting sanctions on Iran.

“Now is a time for political decisions. Now is the time to decide whether — for Iran to decide whether it’s prepared to make those decisions necessary for a mutual return to compliance,” they said.

But notably, they repeatedly took the occasion to bash the “prior administration’s catastrophic error” and “terrible mistake” of withdrawing from the deal — seeming to lay the groundwork for a blame game if talks blow up and Iran’s enrichment only grows.

Former President Donald Trump exited the deal in May 2019 and reimposed strong U.S. sanctions on Iran, driving down its oil exports and sparking tit-for-tat attacks across the Middle East region. His administration repeatedly said its campaign of “maximum pressure” would drive Iran to negotiate a new deal, but Iran refused to meet U.S. officials, even after Biden took office.

With talks expected to resume this week, according to Enrique Mora, the senior EU diplomat who coordinates the talks, one key sign to watch will be whether the U.S. and Iran finally engage directly. Iran’s Foreign Minister Hossein Amir-Abdollahian expressed an openness to it last week if Iran sees a “good deal” in sight, but the senior State Department official said there’s no indication so far the Iranians will sit down. The U.S. has consistently said it’s prepared to meet directly, calling the indirect talks an impediment to reaching a deal so far.

If a deal isn’t reached soon, the U.S. is “ready” to “fortify our response, and that means more pressure — economic, diplomatic, and otherwise,” the senior official said, adding, “We will use the tools that we have to ensure that our interests are preserved and that Iran cannot acquire a nuclear weapon.”

Notably, the Biden administration has refused to say out loud whether that includes supporting the use of force, including by Israel — just that no option is off the table.

The one thing the U.S., European allies in the talks, France, the U.K., Germany, and even China and Russia seem to agree on is that time is running out. But while the senior State Department official called a deal a “big if,” Russia’s envoy was been more buoyant about a resolution.

“My instinct tells me that agreement will be reached soon after mid February,” Ambassador Mikhail Ulyanov tweeted Friday as the talks ended.

That’s in stark contrast to the heavy pessimism in early December after talks finally resumed under Iran’s new government, with its much more hard-line approach.

But the Iranians now are “back in a serious, businesslike negotiation in which there are still significant gaps — so I don’t want to in any way understates those — but we are in a position where… we can see a path to a deal if those decisions are made and if it’s done quickly,” the senior State Department official said.

In the meantime, the U.S. continues to press for the release of four U.S. citizens detained by Iran on specious charges, including father and son Baquer and Siamak Namazi. At 84-years old, Baquer is in particularly vulnerable health and had emergency heart surgery in Tehran in October to clear a “life-threatening” blockage in his carotid artery, according to his lawyer Jared Genser. This month marks his sixth year in Iranian custody, while his son Siamak has been held since October 2015.

“We are negotiating on the release of the detainees separately from the JCPOA, but as we’ve said, it is very hard for us to imagine a return to the JCPOA while four innocent Americans are behind bars or are detained in Iran,” the senior State Department official said.

In addition to the Namazi’s, Iran has detained conservationist Morad Tahbaz and businessman Emad Shargi. All four men are dual U.S.-Iranian citizens whose detentions have been called “hostage diplomacy” by Tehran.