The US and Iran are set to resume talks next week on reviving the Iran nuclear deal, with the Biden administration making clear that restoration remains a priority. Yet as Iran inches ever closer to building a nuclear weapon, time is running.

Last month Tehran announced that it had increased its stockpile of 60% enriched uranium to 25 kilograms, nearly quadruple its June total. As of early December, the UN and Iran have yet to reach agreement over renewed access to key Iranian nuclear sites, and analysts are sceptical of a breakthrough. For Washington, no option has been taken off the table: US officials are thought to be devising a Plan B, in the event that talks collapse.

Under former president Barack Obama’s stewardship, Iran, the US and five other powers signed the historic Joint Comprehensive Plan of Action (JCPOA) in July 2015, halting Iran’s nuclear development in exchange for a lifting of sanctions. The deal, meant to be a stepping stone to broader negotiations on Iran’s regional activities, was not without its critics.

Many complained that it failed to address the regime’s ballistic missile program or advanced centrifuge research and that in lifting sanctions granted Tehran access to increased financing for its regional meddling. In May 2018, Trump pulled out of the deal, opting instead for a “maximum pressure” campaign designed to hobble the Iranian economy and force Tehran back to the negotiating table. Iran responded by restarting aspects of its nuclear program.

During his election campaign and since taking office, Biden had repeatedly made clear that he favors a return to the deal. But analysts fear he has little time left.

US and Iranian officials met in April to discuss pre-conditions for restarting JCPOA. Significant progress was made, but key differences remained. Washington wants Tehran to commit to continuing talks after the re-implementation of the deal, while Iran wants sanctions lifted immediately and a US guarantee that it will not again pull out of the deal.

“On the nuclear side, the issue is more political and less technical,” Abdolrasool Divsallar, senior research fellow and co-lead of Regional Security Initiative at the European University Institute, told TACTICS. “We have more clarity at least when it comes to non-nuclear mechanisms because of safeguards and protocols already in place. Sanctions relief will be more complicated due to the fact that institutions and individuals could remain sanctioned for secondary reasons.”

The looming talks in Vienna will be the first under Iran’s new administration, which means many of the lead negotiators from 2015 are no longer in office on either side. Observers fear a new Iranian negotiating team could hinder talks.

The election of new hard-line President Ebrahim Raisi may also have implications, but it remains less of a concern for US officials. Raisi is likely to call for greater concessions, but he has committed to operate within the agreed-upon framework. Besides, Iran’s foreign policy is set by the supreme leader, Ayatollah Ali Khamenei, who has said he wants a return to the deal.

A key hurdle is that Iran’s nuclear program has made significant progress. While many of its violations are reversible — machines can be dismantled and stored; enriched uranium can be shipped out — knowledge and expertise are not.

As of early December, experts estimate Iran’s breakout time – the time required to enrich uranium for a nuclear bomb – at about two months. This means it will soon be no longer in the US interests to continue negotiations.

It is yet to be seen what, if any, strategy the Biden administration has beyond the current approach. According to U.S. Special Envoy for Iran Robert Malley, all options are on the table. “We have to prepare for a world,” Malley recently said, “where Iran doesn’t have constraints on its nuclear program and we have to consider options for dealing with that, even as we hope that we can get back to the deal.”

If diplomacy fails, Iran is likely to continue to ratchet up its military development, exacerbating regional tensions and reducing the security of the US and its allies. Israel has warned that it is willing to use force to prevent Iran from obtaining a nuclear weapon. In addition to cyber-attacks on Iran’s nuclear facilities, Israel has been charged with the killing of prominent Iranian nuclear scientist, Mohsen Fakhrizadeh, a year ago.

Diplomacy remains the best option. Should talks fail, darkness looms.