Russia said its troops began pulling out of a strategic Ukrainian city on Thursday, a retreat that would represent a humiliating defeat in the grinding war. Ukrainian officials acknowledged Moscow’s forces had no choice but to flee but stopped short of declaring victory in Kherson.

It was difficult to know what was happening in the industrial port city, from which tens of thousands have fled in recent weeks. Some Western observers, including the highest-ranking U.S. military officer, said they believed the Kremlin’s forces have been forced to pull out — though a full withdrawal could take some time.

Ukrainian officials have warned for weeks that any announcement of a Russian retreat should be treated skeptically. They have accused Moscow of plotting to lure Ukrainian troops into an ambush and said Russian soldiers have donned civilian clothes in an effort to melt into the city’s population.

But on Thursday, Ukraine’s armed forces commander-in-chief, Valeriy Zaluzhny, said that “the enemy had no other choice but to resort to fleeing,” since Kyiv’s army has “destroyed logistical routes and supply system, disrupted the system of the enemy’s military command” in the area.

Still, he said that Ukrainian military could not confirm or deny that Russian forces were indeed withdrawing from Kherson as the Russian Defense Ministry reported Thursday. The ministry said there was a “maneuver of units of the Russian group” to the opposite side of the Dnieper River from where Kherson lies, a day after ordering a withdrawal.

But another senior Ukrainian official offered another reason to be wary of any pullback: Presidential adviser Mykhailo Podolyak alleged Russian forces had laid mines throughout Kherson, saying they wanted to turn it into a “city of death.”

Moscow took Kherson soon after the invasion on Feb. 24 — the only provincial capital it has captured — and illegally annexed the wider region of the same name in September.

A forced pullout from the city would mark one of Russia’s worst setbacks and recapturing Kherson could allow Ukraine to win back lost territory in the south, including Crimea, which Moscow illegally seized in 2014. A Russian retreat is also almost certain to raise domestic pressure on the Kremlin to escalate the conflict.