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

There are now 76 Russian battalion tactical groups -- each made up of about 800 to 1,000 troops -- inside Ukraine, all in the south or east of the country, a senior U.S. defense official said Monday. Last week there were 65 battalion tactical groups.

There are about 22 additional battalion tactical groups in the north of Ukraine, most likely refitting and resupplying after being depleted from earlier combat, the official said.

The besieged port city of Mariupol is still under threat of missile and artillery bombardment as Ukrainian forces continue to fight to push back Russian troops, according to the official.

Kyiv and Lviv are under long-range fire, the official said. Russian long-range bombers have hit both cities with air-launched cruise missiles over the last couple of days, the official said.

"Our initial assessment is that they were going after primarily military targets, or what they believed to be military targets," the official said.

The U.S. is sending 18 howitzers -- short cannons used to fire projectiles -- to Ukraine and plans to begin training Ukrainians on the artillery in the coming days, the official said.

-ABC News' Matt Seyler

Police say a total of 269 civilians have been killed in the Ukrainian city of Irpin, located near Kyiv and Bucha.

These fatalities are separate from the 410 bodies recovered in nearby Bucha last week.

-ABC News' Fidel Pavlenko

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.