Ukrainian forces have advanced to within artillery range of a key Russian supply road, as offensives continue in the east and south of the country.
Forward units are now at least 20 kilometres east of the Oskil river - where Russia had tried to establish a line of defence following its defeat in the Kharkiv region last month - and are poised to cross the regional border into Luhansk.
"It is highly likely that Ukraine can now strike the key Svatove-Kremina road with most of its artillery systems, further straining Russia’s ability to resupply its units in the east," Britain’s Ministry of Defence said in an update on Wednesday morning.
Svatove is a supply hub that controls road and rail links between the Russian border and Kreminna, a town further south where Russia is trying to establish a defensive line following the fall of Lyman.
Meanwhile, Serhiy Gaidai, the governor of Luhansk region, said a large number of Russians were pinned down in Kreminna and appeared to be laying minefields in the hope of halting the Ukrainians.
The Ukrainian advance is a political blow to the Kremlin as both towns lie in the Luhansk region, which Russia said it had "fully liberated" in July and which Vladimir Putin formally claimed as part of Russia last week.
He signed the bill finalising the attempted annexation on Wednesday morning.
On Tuesday, Ukrainian forces in the south of the country advanced some 15 miles as Russian forces near Kherson retreated from a threatened encirclement, abandoning positions they had held since March.
Kirill Stremousov, a Russian-installed official in occupied parts of the Kherson region, said on Wednesday morning that the Ukrainian advance had stopped. The claim could not immediately be confirmed.
Unofficial Russian sources reported that the Ukrainians had entered Snihurivka, a town north of Kherson, overnight.
Russian kamikaze drones also attacked Bila Tserkva, a town in the Kyiv region town, and authorities said one person was injured.
The strikes are the deepest penetration of Ukrainian airspace since Russia began using the Iranian-made drones two months ago.
Footage from the scene showed heavy damage to a military barracks there.