Since the start of Russia's invasion, the Ukrainian first lady has been in hiding with her two children. A difficult question her 9-year-old son keeps asking is when the war will end, Olena Zelenska said in an exclusive interview with ABC News.

"Unfortunately, I don't think any Ukrainian would be able to answer that question," Zelenska told "Good Morning America" co-anchor Robin Roberts in her first televised solo interview since the invasion began.

In discussing the state of the conflict nearly 100 days after Russian President Vladimir Putin launched a "special military operation" in Ukraine, Zelenska said 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, 44, said in the interview, airing on "Good Morning America" Thursday. "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 on "Good Morning America" and across ABC News. "Good Morning America" airs at 7 a.m. on ABC.

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

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan