Ukraine has recaptured over 700 sq kms (270 sq miles) of its territory in the east and the south during a lightning counte offensive, a Ukrainian general said on Thursday, offering the first official assessment of the operation.

The gains, if confirmed and held, are a serious blow for Russia which Western intelligence services say has suffered huge casualties. It also would represent a significant boost for Kyiv, which is keen to show its Western backers that it can change the facts on the ground by force and deserves continued support.

Some of the biggest gains appear to have been made this week during a surprise Ukrainian counteroffensive in the Kharkiv region of eastern Ukraine which saw Kyiv's forces make a sudden and deep thrust behind Russian lines.

Brigadier General Oleksiy Gromov told a news briefing that Ukrainian forces had advanced up to 50 kms (30 miles) into Russian lines and recaptured more than 20 villages in the Kharkiv region during an operation, which is ongoing.

Ukrainian leader Volodymyr Zelenskiy had spoken of "good news" on the battlefields of eastern Ukraine the night before and had thanked three brigades for what he called their bravery.

In a late night address to the nation, Zelenskiy had said: "This week we have good news from Kharkiv Oblast. All of you have most likely seen reports about the recent activities of Ukrainian defenders. And I think every (Ukrainian) citizen feels proud of our warriors."

Kharkiv region borders Russia and its main city, Kharkiv, has for months been struck by Russian missiles after Moscow failed to take it in the early stages of its Feb. 24 invasion.