If Russia takes Mariupol, forces could be freed to fight in east: Pentagon update Day 54

The Pentagon has been providing daily updates on the U.S. assessment of the Russian invasion of Ukraine and Ukraine's efforts to resist.

Here are highlights of what a senior U.S. defense official told reporters Monday on Day 54.

Russia 'shaping' the battlefield for renewed offensive

There are now 76 Russian battalion tactical groups (BTGs) inside Ukraine, all of them in the south and east. Each BTG is made up of roughly 800-1,000 soldiers. That's up from 65 BTGs last week, so as many 11,000 troops have crossed the border since then, the senior defense official said.

Last week, the U.S. mostly saw support forces convoying into the Donbas region, but BTGs are Russia's primary fighting units. Artillery, helicopter support and command and control elements have continued to flow in.

In a separate briefing, Pentagon press secretary John Kirby told reporters Monday that while there is currently fighting in Donbas, as there has been for years, defense officials do not believe Russia's new offensive push has begun.

"We believe that the Russians are shaping and setting the conditions for future offensive operations," Kirby said.

That shaping effort appears to be an attempt by Russia to avoid blunders it made during combat operations in northern Ukraine.

"We believe that they are trying to learn from from past mistakes, and you can see that in just the way they are conducting these shaping operations," Kirby said. "They're conducting themselves in ways that we didn't see around Kyiv, for instance."

While Russian forces are "trying to set the conditions for more aggressive, more overt, and larger ground maneuvers in the Donbas," Kirby said Ukrainian troops are not sitting idle.

"We have seen indications in just the last few days that the Ukrainians not only have have defended bravely, but they have been able to secure certain villages and towns in the Donbas," Kirby said.

There are another 22 or so BTGs still in Russia north of Ukraine, most likely refitting and resupplying after being depleted form earlier combat in northern Ukraine, according to a defense official.

The official noted that if Russia takes Mariupol, it would free up close to another 12 BTGs (roughly 8,800-12,000 troops) that could be used for fighting elsewhere in the south or east.

Mariupol

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

Kyiv and lviv under long-range fire

Russian long-range bombers have hit both Kyiv and Lviv with air-launched cruise missiles over the last couple of days, according to the official.

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

During his press briefing, Kirby said the U.S. is still assessing what the Russian's hit in the two cities, but added that most Russian airstrikes are being directed on the east and south of Ukraine, especially Mariupol.

The Moskva

The U.S. can't verify authenticity of videos purportedly showing Russia's Moskva cruiser sinking, "but the images themselves comport with what we had assessed to be the damage done to the ship," the official said.

The official said the U.S. was able to see Russian sailors board lifeboats before the ship sank, but could not confirm how many casualties there might have been.

"It's hard to look at the damage that was done without generally assuming that there were also casualties, that she lost sailors. How many, we just don't know," the official said.

First shipments of new $800 million US aid package arrive

"Based on the last $800 million that the President authorized that we just announced last week, already there have been four flights from the United States arriving into the theater just yesterday," the official said.

US to train Ukrainians on artillery

In coming days, the U.S. plans to facilitate training for Ukrainians on the howitzers it's sending them, according to the official. The 18 howitzers heading to Ukraine will come from a mix of U.S. Army and Marine Corps stocks.

ABC News' Luis Martinez contributed to this report.