Russia-backed separatist forces in eastern Ukraine's Donetsk Oblast claimed Thursday that they have arrested more than 100 captured Ukrainian troops suspected of being involved in "crimes."

"Facts of involvement in crimes have been brought to light following investigators' works. There are already more than 100 people who have been arrested by investigators," Yury Sirovatko, justice minister of the self-declared Donetsk People's Republic, told Russian state-owned television channel Rossiya 24 on Thursday. "They have already been transported to a remand center, where they will await a decision on their fate and a court ruling."

Sirovatko did not elaborate on what types of crimes were allegedly committed. Ukraine did not immediately respond to the claims.

On Wednesday, Sirovatko told Channel One, a Russian state-controlled TV channel, that there are about 2,600 captured Ukrainian servicemen in the region.

Russia on Thursday accused Ukraine of committing war crimes by indiscriminately attacking civilian areas in Ukrainian cities.

The Russian Ministry of Defense claimed that the Ukrainian Armed Forces "launched a massive attack" using ballistic missiles and multiple rocket launchers on residential areas of Kherson in southern Ukraine late Wednesday.

"The indiscriminate missile attack launched by the nationalists targeted kindergartens, schools and various social facilities in residential areas near Ushakova avenue," the ministry said in a statement Thursday. "Russian air defense units have repelled the attack of the Ukrainian troops launched at the residential districts of Kherson."

The ministry also claimed that Ukrainian troops had launched indiscriminate attacks on residential areas of Izyum in eastern Ukraine.

"The Kyiv nationalist regime's indiscriminate attacks on residential areas of Izyum and Kherson are a war crime and a gross violation of international humanitarian law," the ministry added.

Ukraine did not immediately respond to the allegations.

Russian President Vladimir Putin hinted at the possibility of nuclear warfare during his Wednesday address to the council of legislators.

"If someone from outside moves to interfere in the current developments, they should know that they will indeed create strategic threats to Russia, which are unacceptable to us, and they should know that our response to encounter assaults will be instant, it will be quick," Putin said, according to Russian state media.

Putin claimed Russia's response to strategic threats from outside Ukraine would be “immediate."

“We have all the tools to do it, tools that others can't boast of at the moment, but as for us, we won't be boasting,” Putin said.

Putin said that Russia is prepared to use those “tools” if “the need arises," adding that he “would like everyone to be aware of it.” A nuclear attack has been on the table since the onset of the “special military operation” in Ukraine, Putin said. He had ordered his nuclear forces to be put on high alert on Feb 27.

Putin's remarks came as Pentagon press secretary John Kirby announced that “more than half” of the 90 howitzers the U.S. agreed to send to Ukraine were now in the country, adding that around 50 Ukrainian troops have already been trained to operate the weapons.

“We finished up earlier this week, the first tranche of more than 50 trainers that are going to go in and train their teammates,” Kirby said during a press briefing on Wednesday, a moment later adding, “But there was another tranche of more than 50 that we're going to go through training in the same location outside Ukraine.”

The U.S. Department of Defense on Wednesday tweeted pictures of more howitzers “bound for Ukraine” that were being loaded onto US Air Force aircraft. Additional training opportunities on Howitzers and other weapons systems were also being explored, Kirby said.

As U.S. weapons head to Ukraine, Russia is increasing the pace of its offensive in almost all directions, the General Staff of the Ukrainian Armed Forces said on Thursday.

The U.S. is considering the legal aspects of officially listing Russia as a state-sponsor of terrorism, Secretary of State Anthony Blinken told lawmakers on Wednesday. Officials said they haven't yet determined whether Russia's actions meet the legal standard required for the designation, Blinken said.

The designation, called for by Ukrainian President Volodymyr Zelenskyy and Foreign Minister Dmytro Kuleba, would further cripple Russia's trade potential, including bans on defense exports and limits on foreign aid.

-ABC News’ Edward Szekeres, Uliana Lototska and Fidel Pavlenko

President Joe Biden's supplemental budget request to secure more funding for Ukraine could be sent to Capitol Hill as soon as Thursday, according to White House press secretary Jen Psaki.

"In terms of the length or the size, I don't have a number for you at this point," Psaki told reporters at Wednesday's briefing. "But there is plans for this to be a proposal to go through the fiscal year, and it will include -- as our past packages have included -- security or military assistance, humanitarian economic assistance."

Psaki promised that Biden would stick to his word of delivering the supplemental request before the end of the week.

Last Thursday, Biden announced that he would make this request of Congress while announcing the latest $800 million aid package for Ukraine.

“I've almost exhausted the drawdown authority I have that Congress authorized for Ukraine in a bipartisan spending bill last month. In order to sustain Ukraine for the duration of this fight, next week I'm going to have to be sending to Congress a supplemental budget request to keep weapons and ammunition flowing without interruption,” Biden said last week. “My hope and expectation is that Congress would move and act quickly.”

-ABC News' Molly Nagle

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