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

At least one person -- a journalist -- was killed in a rocket attack on a residential building in Kyiv on Thursday evening, ABC News has learned.

Kyiv Mayor Vitali Kilitschko said Friday that rescuers had found the body of a victim amid the rubble.

Radio Liberty, a service of the U.S.-funded Radio Free Europe/Radio Liberty, reported that one of its employees, Vira Gyrych, was killed when a Russian missile hit her apartment in the Ukrainian capital on Thursday. Her body was found beneath the wreckage Friday morning, according to the report.

Gyrych had worked as a journalist and producer for Radio Liberty's Kyiv bureau since 2018. Prior to that, she worked for leading Ukrainian television channels, according to Radio Liberty.

"The editorial staff of Radio Liberty expresses its condolences to the family of Vira Gyrych and will remember her as a bright and kind person, a true professional," Radio Liberty said in its report.

Israeli Ambassador to Ukraine Michael Brodsky also confirmed Gyrych's death in a Twitter post, saying she was a former employee of the Israeli embassy in Kyiv.

Thursday's rocket attack came as United Nations Secretary-General Antonio Guterres visited Kyiv. Five Russian missiles flew into the city, according to Ukrainian President Volodymyr Zelenskyy. At least 10 people were injured, including four who were hospitalized, according to the Kyiv City Council.