Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Over half of Sieverodonetsk is now likely occupied by Russian forces, including Chechen fighters, the U.K. Ministry of Defense said Wednesday in an intelligence update.

The ministry said fighting intensified in the streets of the key eastern Ukrainian city on Monday and Tuesday, "with Russian forces pushing closer to the town centre." Sieverodonetsk is located in the war-torn Luhansk Oblast of eastern Ukraine's disputed Donbas region.

"Russian ground operations remain tightly focused, with the weight of fire power concentrated within a small sector of Luhansk Oblast," the ministry said.

"Beyond the Donbas, Russia continues to conduct long-range missile strikes against infrastructure across Ukraine," the ministry added. "The strategically important bridge links Ukraine with Romania and with Ukraine's ports on the Danube, which have become critical to Ukrainian exports after the blockade of Ukrainian Black Sea ports by Russia."

The U.S. will provide Ukraine with "more advanced rocket systems and munitions," President Joe Biden confirmed in a New York Times op-ed published Tuesday.

The systems "will enable them to more precisely strike key targets on the battlefield in Ukraine," he wrote.

Russian Foreign Minister Sergei Lavrov warned last week that providing more advanced rockets would be a new "unacceptable escalation" because they could hypothetically be used to strike within Russian territory.

But Biden told reporters Monday that the U.S. will not "send to Ukraine rocket systems that can strike into Russia," and he and the administration are making efforts to emphasize that these new rocket systems will be used by the Ukrainians on the battlefield in their own country.

-ABC News' Conor Finnegan

Russia's invasion of Ukraine has left 5.2 million kids in need of humanitarian help, according to UNICEF.

At least 262 children have been killed and another 415 have been hurt since Feb. 24, UNICEF said.

Nearly two-thirds have been displaced, UNICEF said.

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.

Haidai is urging residents to stay inside and to wear protective face masks.

Haidai also said Russian forces have made significant gains and are in control of "most" of Severodonetsk.

He said about 60% of homes are completely destroyed and the city's critical infrastructure is nearly completely destroyed. Ongoing shelling is preventing civilians from evacuating.