Ukrainian forces liberated a key village in the southern region of Kherson on Tuesday.
It came amid reports Russian troops had abandoned another area so rapidly that they left the bodies of their comrades in the streets.
A video showing the 35th marine brigade hoisting a Ukrainian flag above Davydiv Brid was posted by the country’s defence ministry. Several other nearby villages were recaptured.
President Vladimir Putin’s men have already been forced to retreat in Ukraine’s north-east and are now being pushed back in the south as well.
Despite the Kremlin’s apparent political bravado, the picture on the ground underscored the disarray of Putin’s attempt to annex four Ukrainian regions, while war was raging in all four.
The annexation has no legitimacy under international law and Ukraine’s President Volodymyr Zelensky has declared them null and void.
Over the weekend, the Russians pulled back from Lyman, a strategic eastern town being used as a logistics and transport hub to avoid being encircled.
Two days later, an Associated Press team reporting from the city saw at least 18 bodies of Russian soldiers still on the ground.
President Joe Biden spoke to Zelensky and assured him the US would never recognise attempts to establish new Russian borders. They also discussed $625m (£550m) of US aid, including Himars rocket launchers.
Russia still controls Kherson city, the regional capital, in the south. But its grip is looking increasingly shaky on the entire area north of the River Dnieper, known as Dnipro in Ukrainian.
Ukraine’s defence ministry said on social media as residents filmed soldiers walking through the streets: “The Ukrainian flag flies again over the village of Davydiv Brid.”
Putin has threatened to use tactical nuclear weapons. But Foreign Secretary James Cleverly said that “would not go without a response”.
He declined to discuss “the nature or the threshold” but said: “What we have seen in Vladimir Putin’s decision-making is that he has made just so many strategic errors.
“Increasingly what we need to do is we need to make it very clear that his sequence of strategic errors has got to stop.”
Cleverly also vowed to “continue to support the Ukrainians in the defence of their homeland”.