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

At a press availability Wednesday, Secretary of State Antony Blinken lauded what he called a "significant new security assistance package to arm Ukraine with additional capabilities and advanced weaponry," saying it was "precisely what they need to defend themselves against the ongoing Russian aggression."

While Blinken noted the U.S. was leveling up the military equipment it was supplying to Ukraine, he described it as an extension of the administration’s longstanding game plan.

"[The package] includes more advanced rocket systems so that they can strike key targets on the battlefield in Ukraine from longer distances," Blinken said. "This is a continuation of a strategy that began even before Russia's invasion. We move quickly to send Ukraine significant amounts of weapons and ammunition so that they can repel Russia's aggression, and in turn, can be in the strongest possible position at any negotiating table that may emerge."

NATO Secretary General Jens Stoltenberg, who was also at the press availability, praised the latest round of assistance, calling it "a demonstration of real U.S. leadership."

But he added, "At the same time, we must prevent the conflict from escalating."

Blinken was pressed on how the U.S. could ensure that the rocket systems would not intensify or expand the war.

"With regard to weapons systems being provided, the Ukrainians have given us assurances that they will not use these systems against targets on Russian territory. There is a strong trust bond between Ukraine and the United States, as well as with our allies and partners," he answered.

Blinken added that none of this should surprise the Kremlin.

"Throughout this aggression, indeed, even before -- President [Joe] Biden was very clear with [Russian] President [Vladimir] Putin about what the United States would do if Russia proceeded with its aggression, including continuing to provide security systems that Ukraine needs to defend itself against the Russian aggression," he said. "So we have done exactly what we said we would do."

-ABC News' Shannon Crawford

Russian shelling in eastern Ukraine's Kharkiv Oblast over the past 24 hours left four civilians dead, the regional governor said Wednesday.

A woman was killed in the village of Zolochev, while a 12-year-old boy died in the village of Ivanivka, northwest of Kharkiv city. Two more people were killed in the village of Shestakove, northeast of Kharkiv city. Seven other civilians were injured in the region, according to Kharkiv Oblast Gov. Oleg Synegubov.

"The enemy is acting insidiously, striking at civilians and civilian infrastructure," Synegubov said in a statement via Telegram. "But he will answer for all his crimes! Ukraine will win!"

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