"The Ukrainians are trying to take Kherson back, and we would argue that Kherson is actually contested territory again," the Pentagon official told reporters.

"We can't corroborate exactly who is in control of Kherson but the point is, it doesn't appear to be as solidly in Russian control as it was before," said the official, who spoke on condition of anonymity.

If the Ukrainian forces manage to regain control of the strategic city located at the mouth of the Dnieper, Russian troops around Mikolaiv would be "sandwiched" between Ukrainian forces defending Mikolaiv and those in Kherson, the official said.

"That would that would make it very, very difficult for them to make any kind of ground movement on Odessa," the main seaport still held by the Ukrainians.

"That would be a significant development, no question about that, in terms of the southern part of the war," he said.

The Pentagon said there was also "very heavy fighting" to the northwest of the capital Kyiv where "the Ukrainians are trying hard to dislodge the Russians from Bucha and Irpin."

Russia is meanwhile mobilizing forces from separatist territories of Georgia to deploy in Ukraine, the official said.

"We've seen our first indications that they are trying to send in some reinforcements from Georgia," the senior official told reporters.

Following a flashpoint conflict with Georgia in 2008, Russia recognized the independence of two pro-Russian Georgian separatist regions, Abkhazia and South Ossetia and establishing military bases there.