The U.S. has assessed that the Ukrainian port city of Mariupol remains contested, and that Russian airstrike activity remains focused there and on the Donbas region, a senior U.S. defense official said Thursday.

Russia now has 85 battalion tactical groups, each made up of roughly 800 to 1,000 troops, inside of Ukraine, the official said. More of these groups are headed to the Donbas region, the official said.

-ABC News' Luis Martinez

Mariupol Mayor Vadym Boychenko told ABC News he thinks his city will hold out, saying Russian forces have "been fighting our boys for 57 days and they still can’t win."

The mayor's comments come hours after Russian President Vladimir Putin declared that his siege of Mariupol had been a success, congratulating his defense minister and thanking Russian troops. Putin also ordered troops to abandon their assault on the Azovstal Iron and Steel Works plant, the last holdout for Ukrainian troops in the port city.

Boychenko said 100,00 civilians remain, including 1,000 in the steel plant.

President Joe Biden also pushed back on Putin's claim that Russia has control over Mariupol, saying at a news conference Thursday, "There is no evidence yet that Mariupol has completely fallen."

-ABC News' James Longman

Ukrainian Deputy Prime Minister Iryna Vereshchuk said 19 Ukrainians, 10 military and nine civilians, have been released from Russian captivity in the second prisoner swap this week.

-ABC News' Alexandra Faul

Russian President Vladimir Putin declared on Thursday that his siege of Mariupol had been a success, congratulating his defense minister and thanking Russian troops.

"The completion of the combat work to liberate Mariupol is a success," Putin said. “I congratulate you. Convey words of gratitude to the troops."

Putin’s claim of victory came as he ordered troops to abandon their assault on the Azovstal Iron and Steel Works plant, the last holdout for Ukrainian troops in the port city.

A Ukrainian commander of the regiment at the site said Ukrainian troops there are ready to surrender, if their safety can be guaranteed by a third party and they are allowed to take the bodies of their dead with them.

Ukrainian President Volodymyr Zelenskyy's office called for negotiations inside Mariupol to get anyone left in the factory out of the area alive.

At a news conference Thursday, President Joe Biden pushed back on Putin's claim that Russia has control over Mariupol.

"There is no evidence yet that Mariupol has completely fallen," Biden said.

-ABC News’ Tanya Stukalova, Iryna Hnatiuk, Molly Nagle and Yulia Drozd

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan