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

Russian forces seized the town of Kreminna in eastern Ukraine's Luhansk Oblast on Monday, following fierce fighting in the streets, according to the regional governor, Serhiy Haidai.

"Currently, control over the city of Kreminna is lost, street fights are taking place," Haidai said in a statement via Telegram on Monday.

He warned Kreminna residents not to try to escape. Russian forces opened fire on civilians who were trying to leave the town in their own car on Monday morning. Four of them died and a fifth was seriously injured and remains at the scene, according to Haidai.

"Doctors cannot reach her due to endless shelling," he said.

Since 2014, Russia-backed separatist forces have controlled two breakaway republics of the Luhansk and Donetsk oblasts in the disputed Donbas region of eastern Ukraine. The separatists have been fighting alongside Russian troops to seize more territory there, after Russia launched its invasion of Ukraine on Feb. 24. Now, the Russian military is said to be refocusing its offensive in the Donbas after withdrawing troops from northern Ukraine.

Humanitarian corridors to evacuate civilians will not reopen in eastern Ukraine on Monday due to continuous bombardment from Russian forces, according to Ukrainian Deputy Prime Minister Iryna Vereshchuk.

"In violation of international humanitarian law, Russian occupiers do not stop blocking and shelling humanitarian routes," Vereshchuk said in a statement via social media on Monday. "Therefore, for safety reasons, a decision was made today not to open humanitarian corridors. We will make all our efforts to make the humanitarian corridors work again as soon as possible."

Russian missiles destroyed 16 Ukrainian military facilities overnight, including five command headquarters, a fuel depot and three ammunition depots, as well as concentrations of Ukrainian military "personnel and equipment" in several areas of eastern Ukraine, according the Russian Ministry of Defense.

The Ukrainian deputy premier issued an appeal to Russian leadership, urging that evacuation routes for civilians be allowed to open from Mariupol, a hard-hit port city in the besieged Donetsk Oblast, to Berdyansk, a port city in the Zaporizhzhia Oblast. She also demanded an "urgent humanitarian corridor" for civilians, including women and children, who are sheltering in the territory of the Azovstal iron and steelworks plant in Mariupol.

"Your refusal to open these humanitarian corridors will in the future be a basis for bringing all involved to prosecute for war crimes," Vereshchuk said.

At least seven people were killed and 11 others, including a child, were injured Monday morning in missile strikes across the western Ukrainian city of Lviv, according to local officia;s.

Lviv Oblast Gov. Maksym Kozystkiy said at a press conference that Russian missiles struck four targets -- three Ukrainian military infrastructure facilities and a tire service shop -- all of which suffered significant damage.

In a statement via social media, Lviv Mayor Andriy Sadovyi described the missile strikes as "powerful," saying they damaged or destroyed about 40 cars. Emergency services were responding to the deadly blasts, according to Sadovyi.

ABC News was at the scene of the burning tire service shop on the outskirts of Lviv, where firefighters were trying to extinguish the flames.

The strike also shattered the windows of a nearby orphanage as well as a hotel, where Sadovyi said evacuated Ukrainians are sheltering.

Lviv, a strategic city close to Ukraine's border with Poland, has been considered a safe haven for Ukrainians fleeing the Russian invasion. The city has been largely spared from the relentless bombardment and heavy fighting seen across much of the country since Feb. 24.

-ABC News' Brian Hartman, Max Uzol and Yuri Zaliznyak

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn