Ukrainian troops are already putting U.S. howitzers into the fight against Russia, Defense Secretary Lloyd Austin told senators Tuesday.

During a Senate Appropriations Committee hearing on the 2023 defense budget, Sen. John Hoeven, R-N.D., asked Austin whether Ukrainian forces have the training they need to use the 90 American 155mm artillery systems the U.S. is sending to aid in the battle, especially in the eastern part of the country.

"They're using them as we speak," Austin said. "As you may know, we took a number of troops out and trained them up very quickly on 155s, put them back into action, and they are employing those weapons systems now."

A senior U.S. defense official told reporters Monday that some 200 Ukrainians have now been trained to use the M777 artillery pieces. The training was done by both U.S. and Canadian forces outside of Ukraine. Another 50 Ukrainians will begin howitzer training at an undisclosed location later this week.

The Ukrainian troops leaving the country to learn the American systems are already artillerymen, so training takes only about a week, according to U.S. officials. When they return they teach others what they learned.

Pentagon press secretary John Kirby told reporters on Monday that a "significant majority" of the 90 howitzers committed to Ukraine have arrived in country. The U.S. is also sending 184,000 artillery rounds for the weapons.

-ABC News' Matt Seyler

Civilians, including women, children and the elderly, trapped for weeks inside the Azovstal Iron and Steel Works plant in Mariupol have arrived safely in Zaporizhzhia, according to the United Nations.

A woman evacuated from the plant told ABC News in Russian, "They bombed us every day ... at night, in the morning."

"It was horrible. It is just the complete elimination of all people," she said.

"I'm relieved to confirm that the safe passage operation from Mariupol has been successful," tweeted Osnat Lubrani, the U.N.'s resident coordinator and humanitarian coordinator in Ukraine. "The people I travelled with told me heartbreaking stories of the hell they went through. I'm thinking about the people who remain trapped. We will do all we can to assist them."

Lubrani said 101 civilians were brought out of the plant and another 58 civilians from the Mariupol area joined the evacuation. The youngest was 6 months old, she said.

"It was moving and horrifying to sort of witness people that have been locked in darkness, living under incessant shelling, coming and seeing the sky for the first time in two months," Lubrani said at a Tuesday briefing.

Lubrani said some were too frightened to come out. She said they were in contact with about 30 civilians who chose not to leave, because they couldn't leave without going back into the city to find out the fate of their loved ones.

For others, Lubrani said, it was physically challenging to leave.

They had also lacked proper access to water, food and sanitation, she noted.

Many more people remain trapped at the plant. The sprawling industrial site is the last holdout for the Ukrainian resistance in Mariupol, as Russian forces accelerate their efforts to fully capture city. The Mariupol City Council has previously said there are at least 1,000 people, including Ukrainian troops, on the grounds of the Azovstal plant.

-ABC News' Zoe Magee and Christine Theodorou

Parts of Lviv are without power following missile strikes on Tuesday, Mayor Andriy Sadovyi said.

Two power substations were damaged as a result of the strikes on the western Ukrainian city, near the Polish border.

It's not clear if there are any casualties.

Ukraine's lead prosecutor, Iryna Venediktova, said Russian President Vladimir Putin should "absolutely" be prosecuted for the war crimes she says occurred in the town of Irpin and surrounding communities.

"He’s the main war criminal of the 21st century," she said.

"We all know who started this war. And this person is Vladimir Putin," she said.

Venediktova said the first phase of the war crimes investigation in Irpin has ended. She said investigators found evidence of rape, torture and the use of banned weapons of war in the city.

-ABC News' Marcus Moore

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