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 main road into the Sieverodonetsk pocket likely remains under Ukrainian control but Russia continues to make steady local gains, enabled by a heavy concentration of artillery," the ministry said. "This has not been without cost, and Russian forces have sustained 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.

"Crossing the Siverskyy Donets River -- which is a natural barrier to its axes of advance –- is vital for Russian forces as they secure Luhansk Oblast and prepare to switch focus to Donetsk Oblast," the ministry added. "Potential crossing sites include between Sieverodonetsk and the neighbouring town of Lysychansk; and near recently-captured Lyman. In both locations, the river line likely still remains controlled by Ukrainian forces, who have destroyed existing bridges."

Sievierodonetsk and Lysychansk are the last major cities in the Luhansk area still controlled by Ukraine.

"It is likely Russia will need at least a short tactical pause to re-set for opposed river crossings and subsequent attacks further into Donetsk Oblast, where Ukrainian armed forces have prepared defensive positions," the ministry added. "To do so risks losing some of the momentum they have built over the last week."

In an exclusive interview with ABC News, Ukrainian first lady Olena Zelenska spoke about the state of the ongoing conflict with Russia and where the Ukrainian people currently stand as a country.

In her first televised solo interview since the invasion began, Zelenska, 44, told "Good Morning America" co-anchor Robin Roberts that conceding territory to Russia won't stop the war.

"You just can’t concede…parts of your territory. It’s like conceding a freedom," Zelenska said. "Even if we would consider territories, the aggressor would not stop at that. He would continue pressing, he would continue launching more and more steps forward, more and more attacks against our territory."

The interview with Zelenska will air Thursday, June 2, on "Good Morning America" and across ABC News. "GMA" airs at 7 a.m. on ABC.

President Joe Biden issued a written statement Wednesday announcing his new package of security assistance to Ukraine, which the administration announced Tuesday night.

Biden wrote in a New York Times Op-Ed Tuesday that he'll provide longer-range missiles to Ukraine. His new statement says the systems will be "HIMARS with battlefield munitions." (The U.S. is providing munitions that travel only 50 miles, for use in eastern Ukraine, as opposed to the maximum the HIMARS system can handle, which is around 190 miles, which could strike into Russia.)

This is part of a $700 million security assistance package. Biden said, "This new package will arm them with new capabilities and advanced weaponry, including HIMARS with battlefield munitions, to defend their territory from Russian advances."

-ABC News' Ben Gittleson

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

Russia's invasion of Ukraine has already engaged around 330,000 Russian personnel, Brigadier General Oleksiy Hromo of the General Staff of the Ukrainian Armed Forces told local media on Thursday.

“[Russian] groups at various axes are close to 150,000 strong. If we add their air and sea components, there are about 220,000 military personnel,” Hromo said. Russia also deployed “units of the national army combat reserve, the federal service of guard forces, and mobilization units,” the Brigadier General added.

Hromo's sobering account came on the back of claims made by a top Ukrainian official earlier on Wednesday that revealed mounting Ukrainian casualties. Up to 1,000 Ukrainian soldiers are being killed or wounded each day in the Donbas region of eastern Ukraine, with 200 to 500 killed on average and many more wounded, said David Arakhamia, who heads the presidential faction in the Ukrainian parliament.

In early June, Ukrainian President Volodymyr Zelenskyy said daily fatalities among Ukrainian ranks at the Donbas front were between 60 to 100 troops.

Arakhamia, one of Zelenskyy's closest advisers who oversees Ukraine's stalled negotiations with Russia, has been leading a Ukrainian delegation in Washington this week in a bid to lobby the Biden administration and Congress.

Arakhamia's team want to see the pace of weapons shipments increased and is pushing for a designation of Russia as a state sponsor of terrorism.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Yulia Drozd and Max Uzol