The territorial destiny of Donetsk is the key issue preventing the conclusion of a peace deal between Ukraine and Russia, U.S. Secretary of State Marco Rubio said Wednesday.

“The one remaining item … is the territorial claim on Donetsk. There is active work going to try to see if both sides’ views on this can be reconciled,” Rubio told a U.S. Senate Foreign Relations Committee meeting.

“It’s still a bridge we haven’t crossed. It’s still a gap, but at least we’ve been able to narrow down the issue set to one central one, and it will probably be a very difficult one.”

Ukraine’s Donbas, which consists of the coal-rich Donetsk and Luhansk regions in the east of the country, has since 2014 been the site of an armed conflict between the Ukrainian military and Russia-backed separatists. According to open-source maps of the conflict, Russian forces now control about 80 percent of the Donbas region.

Annexing the Donbas has been one of the maximalist war goals of Russian President Vladimir Putin, who in December said Russia will seize it “one way or another” if Ukraine doesn’t give it up voluntarily.

Ceding Donbas was also one of the points in a 28-point plan circulated by U.S. President Donald Trump’s team, which drew criticism from Ukrainian and European officials as heavily skewed in Russia’s favor. An updated proposal watered down some of the more pro-Russian aspects of the initial plan.

Ukrainian President Volodymyr Zelenskyy has repeatedly vowed that Ukraine will not give up Donbas as part of any ceasefire deal, as that would give Putin a springboard for a future invasion.