Iran, the U.S. and other world powers are nearing a deal to revive the 2015 nuclear accord, although negotiators are still wrangling over significant final demands from Tehran, including the scope of sanctions relief.

An agreement could be finalized in Vienna within the next couple of days, said officials involved in the talks. President Biden has made restoring the agreement a top foreign-policy goal. The White House views an agreement restraining Iran’s nuclear program as key to Middle East stability, allowing the U.S. to focus on China and Russia.

An agreement would set out the steps Iran and the U.S. must take to return to compliance with the 2015 deal, which imposed tight but temporary limits on Iran’s nuclear program in exchange for suspending international sanctions. The U.S. pulled out of the deal in May 2018 and reimposed sweeping sanctions on Iran, which brought most international trade with Iran to a standstill.

While the terms of a restored deal would be almost identical to the 2015 pact, Iran’s “breakout time”—the duration needed to amass enough nuclear fuel for a bomb—could fall to as low as six months, down from about a year in the original deal, U.S. officials say. That is because of expertise that Iran has gained through its nuclear work since the U.S. exited the agreement.

Any deal would likely need final signoff by the leaders of the countries involved: the U.S., the U.K., France, Germany, Russia, China and Iran. Among the most important unresolved issues is how many U.S. sanctions will be lifted, including those on the office of Iranian Supreme Leader Ayatollah Ali Khamenei and Iran’s Revolutionary Guard Corps.

“The U.S. must prove its will to lift major sanctions,” Iran’s President Ebrahim Raisi said Monday.

U.S. and European officials have indicated that they want a deal by the end of February, because of advances in Iran’s nuclear work. Western diplomats have warned they could leave the negotiating table in coming days if an agreement isn’t within reach.

“The Iranian leadership now has a choice. This is the moment of truth,” German Chancellor Olaf Scholz said Saturday at the Munich Security Conference.

Iranian foreign-ministry spokesman Saeed Khatibzadeh said Monday that “the remaining issues are…the most difficult, serious and key ones.”

White House spokeswoman Jen Psaki said Friday that negotiations had progressed substantially, “but nothing is agreed to until everything is agreed.”

Negotiations began last April, nearly three years after then-President Donald Trump withdrew the U.S. from the nuclear deal in 2018, saying the pact gave Iran too much for too little in return.

Biden administration officials have said they hope that a restored deal could provide a platform for follow-up talks about Iran’s missile program and the network of militias that it supports throughout the Middle East. Iran, which says its nuclear work is for entirely peaceful purposes, has repeatedly said it has no interest in further talks.