Hundreds of Ukrainian troops may have been captured by advancing Russian units or disappeared during Ukraine’s chaotic retreat from the eastern city of Avdiivka, according to senior Western officials and soldiers fighting for Ukraine, a devastating loss that could deal a blow to already weakening morale.

The Russian capture of Avdiivka has emerged as a significant symbolic loss for Ukrainian troops, a sign of the battlefield impact of the failure of the U.S. Congress, so far, to approve more military assistance as dwindling supplies of artillery shells make it even harder to hold the line.

Estimates of how many Ukrainians were captured or missing vary, and a precise count may not be possible until Ukraine solidifies new defensive lines outside the city. But two soldiers with knowledge of Ukraine’s retreat estimated that 850 to 1,000 soldiers appear to have been captured or are unaccounted for. The Western officials said that range seemed accurate.

American officials say the loss of Avdiivka is not a significant strategic setback, arguing that Russian gains in eastern Ukraine will not necessarily lead to any collapse of Ukrainian lines and that Moscow is unlikely to be able to follow up with another major offensive.

But the capture of hundreds of soldiers could change that calculus. American officials have said in recent days that morale was already eroding among Ukrainian troops, in the wake of a failed counteroffensive last year and the removal of a top commander. Because of those problems, the officials said, Ukraine’s military has struggled with recruitment.

Ukrainian military officials have said they want to mobilize up to 500,000 more people, but the request has met political resistance and is stalled in Parliament. The capture of hundreds of soldiers, especially those with battlefield experience, would make the need for more troops more acute and complicate the effort to recruit more.