Russian President Vladimir Putin's "special military operation" into neighboring Ukraine began on Feb.
24, with Russian forces invading from Belarus, to the north, and Russia, to the east.
Ukrainian troops have offered "stiff resistance," according to U.S. officials.
The Russian military has since launched a full-scale ground offensive in eastern Ukraine's disputed Donbas region, capturing the strategic port city of Mariupol and securing a coastal corridor to the Moscow-annexed Crimean Peninsula.
Russian forces have taken control of most of Sieverodonetsk in eastern Ukraine, the U.K. Ministry of Defense said Thursday in an intelligence update
The major road into Sieverodonetsk is likely still under Ukrainian hands, but Russia is making steady local gains, the ministry said. "Russian forces suffered losses in the process."
Sieverodonetsk, an industrial hub, is the largest city still held by Ukrainian troops in the contested Donbas region of Ukraine's east, which comprises the self-proclaimed republics in Luhansk and Donetsk oblasts.
Russian air strikes hit a tank with nitric acid at a chemical plant in Severodonetsk, a city in eastern Ukraine, according to Luhansk's regional governor, Serhiy Haidai.