Iranian and U.S. officials are entering a crucial week of negotiations to restore the 2015 nuclear deal, with significant differences remaining on several key issues and new concerns that Russia’s invasion of Ukraine could complicate the talks.

Iran’s chief negotiator, Ali Bagheri-Kani, arrived in Vienna Monday morning with positions that could prove difficult to bridge with his Western counterparts, diplomats said. With Iran continuing to expand its nuclear work, Western diplomats have warned that the negotiations could collapse if a deal isn’t reached this week.

American and Iranian officials say the differences include the scope of American sanctions relief, continued Iranian demands that the U.S. provide stronger guarantees that it won’t again exit the deal and the U.S. push to ensure that a prisoner swap occurs alongside restoring the nuclear pact.

Another issue has emerged as a critical last-minute obstacle: Iran’s efforts to shut down an International Atomic Energy Agency probe into nuclear material found in Iran.

The Vienna talks are aimed at agreeing on the steps Iran and the U.S. must take to return to compliance with the 2015 deal, which strictly but temporarily limited Iran’s nuclear work in exchange for a lifting of most international sanctions on Tehran.

The Trump administration withdrew the U.S. from the deal in 2018, saying it didn’t do enough to close Iran’s pathway to a nuclear weapon. A year later, Iran started expanding its nuclear work again. Tehran says its nuclear program is for purely peaceful purposes.

There are concerns among diplomats that—with Europe, Washington and Moscow at loggerheads over Russia’s invasion of Ukraine—the conflict there could weaken unity among the powers negotiating with Iran. Along with France, the U.K., Germany and China, Russia is also participating in the talks and, with China, has played an important role in nudging Iran toward a deal.

Western diplomats also worry that Tehran will harden its position at a critical moment.

The Ukraine crisis has driven oil prices above $100 a barrel for the first time in eight years, piling political pressure on President Biden. A nuclear could eventually result in up to a million barrels of Iranian oil flowing back into global markets, potentially easing prices.

“The war likely gives Iran a stronger hand in the negotiations—and that raises the risk Tehran will overplay it,” said Henry Rome, a Middle East analyst specializing in Iran at Eurasia Group, a political-risk consulting firm

On Monday, Russian officials played down concerns about the Ukraine crisis undercutting nuclear talks, with Moscow’s chief negotiator, Mikhail Ulyanov, saying the situation “has no relation to the Vienna talks.”

The 2015 nuclear deal was negotiated against the backdrop of Russia’s annexation of Crimea, which led the West to impose significant sanctions on the country.

A senior U.S. State Department official said Friday that there were days left to close the remaining differences.

“Final decisions have to be taken this week—either we have a deal or we do not,” an official from one of the European countries at the talks said Monday. “The context of the current international crisis means the window of opportunity is closing.”