Russia has said that retreat from the key city of Kherson has begun, but Ukraine remains cautious.
The withdrawal from the city of Kherson is Russia's "biggest setback of the war" – it is the only regional capital Russian forces had seized during the eight-month war.
"This has got to be bad for morale," Professor Michael Clarke, a former director of the Royal United Services Institute (RUSI), the defence and security think tank, told Forces News.
"Kherson is the only significant city the Russians took in February. The Kremlin can dress this up in whatever way they want.
"The soldiers themselves know that they were occupying a city, they weren't popular in the city, and there were a lot of civilian attacks on them, which became quite vicious and difficult.
"And now they're withdrawing. The Russian military is already, if you look at Telegram channels in the first 48 hours after this announcement was made, alive with the Russian military feeling disgruntled by this setback.
"Which is the biggest setback of the war from their point of view."
John Spencer, a retired US Army Major and urban warfare expert, said on Twitter: "A great day for Ukraine. Kherson city is being liberated. The only regional capital Russia was able to seize the entire war. A major loss for Russia."
The city, with a pre-war population of 280,000, is the only regional capital to be captured by Russian forces since the invasion began on 24 February. More than 70,000 residents were evacuated in late October.
Prof Clarke believes the Russians "will want this to be a properly organised withdrawal".
"They've got a lot of troops, maybe 40,000 troops to get out. It won't be a rout. They will want it to be a withdrawal in good order. And they've already laid a lot of the foundations for that and it has already been occurring."
He went on: "But they will be very lucky if they get all 40,000 troops onto the east bank of the Dnipro river without taking some losses.
"Because the Ukrainians, for sure, have the means to attack them as they withdraw. Withdrawal in contact... is the most difficult and dangerous military manoeuvre in the book."
Watch: Russia "yet again" fails at one of its main objectives, Defence Secretary Ben Wallace says.
The former RUSI director did highlight that it was "possible" that the Russian withdrawal was all part of a ruse to "bring the Ukrainians into a killing zone".
However, he doesn't think that would work.
"The Ukrainians are already thinking about that," he said. "They are not so foolish as to go rushing in. I think what the Ukrainians will do is they will probe fairly gently, or carefully, towards Kherson city.
"The big issue is not really what happens in the city. I think the Ukrainians will stay out of the city for a while. The big issue is what happens further north."
He added: "I think this battle will turn, not so much on what happens in Kherson over the next week, but what happens in Nova Kakhovka in the next two weeks."