Ukraine's armed forces have recaptured large swaths of territory and are making "significant gains" against Russia's occupation of the northwest region of Kharkiv, the U.K.'s defense ministry said in an intelligence briefing Sunday.

Russian forces have likely “withdrawn units from the area,” but fighting continues around the strategically important cities of Kupiansk and Izyum, it said in its daily update on the war in Ukraine, posted to Twitter.

Ukraine's government claims Russia's retreat from Kharkiv is a major turning point in the six-month-old conflict, as thousands of Russian soldiers abandoned their weapons and ammunition stockpiles to flee the Ukrainian advance, they said.

Some Western experts have described the withdrawal as the worst defeat for Russian forces since they were forced back from the capital Kyiv in March.