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

There are now 92 operational Russian battalion tactical groups -- each made up of about 700 to 1,000 troops -- inside Ukraine, a senior U.S. defense official said Thursday.

Russia continues to suffer logistical problems, as it has since early in the invasion. This, on top of the Ukrainian defense, is slowing their progress.

"They're only able to sustain several kilometers or so progress on any given day just because they don't want to run out too far ahead of their logistics and sustainment lines," the official said.

The official added, "We would assess that Russian forces are making slow and uneven, and frankly, we would describe it as incremental progress, in the Donbas."

More than 54 of the 90 howitzers the U.S. is sending Ukraine have arrived in the country, the official said.

The first batch of 50 Ukrainians taken out of the country for training on U.S. artillery systems is back in Ukraine, where they can teach others what they've learned, the official said. A second group of 50 Ukrainians has begun its six days of training, the official said.

-ABC News' Matt Seyler

President Joe Biden is asking Congress for a total of $33 billion in supplemental aid for Ukraine over the next five months, administration officials previewed in a Thursday morning call ahead of the president's remarks.

Over $20 billion of the $33 billion will be for military and other security systems.

"The cost of this fight is not cheap. But caving to aggression is going to be more costly," Biden said in remarks later in the morning.

Biden stressed, "We're not attacking Russia -- we're helping Ukraine defend itself against Russian aggression."

The administration is also asking for an additional $8.5 billion in economic assistance to help provide basic services to the Ukrainian people.

Part of the package also includes targeted funding to address economic disruptions in the U.S. as a result of the war in Ukraine, like helping increase U.S. production of wheat and soybeans, "and funding to allow the use of the Defense Production Act to expand domestic production of critical reserves - of reserves of critical minerals and materials that have been disrupted by Putin’s war and are necessary to make everything from defense systems to cars," a senior administration official said.

Biden insisted that, despite Russia's claims, the U.S. is not fighting a proxy war.

"It shows the desperation that Russia is feeling about their abject failure in being able to do what they set out to do in the first instance," Biden said.

-ABC News' Justin Gomez and Armando Tonatiuh Torres-García

The Russian Navy still has the ability to strike coastal targets in Ukraine, even after the "embarrassing losses" of two warships, according to the U.K. Ministry of Defense.

In an intelligence update posted Thursday, the ministry said approximately 20 Russian naval vessels, including submarines, are currently in the "Black Sea operational zone." But the ministry said Russia isn't able to replace the missile cruiser Moskva because the Bosphorus strait remains closed to all non-Turkish warships.

The Moskva, the flagship of Russia's Black Sea fleet, sunk in the Black Sea earlier this month while being towed to port after a fire onboard, according to the Russian Ministry of Defense. Ukrainian officials, however, claimed that ship was struck by Ukrainian missiles, which the Russian defense ministry has denied.

Russia also lost the landing ship Saratov, which was destroyed by explosions and fire on March 24.

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.