Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

Ukrainian President Volodymyr Zelenskyy Saturday demanded the release of Melitopol Mayor Ivan Fedorov, who Ukrainian officials allege was kidnapped by Russian forces.

"The city community is demanding his release. I am grateful to every Melitopol resident for this resistance," Zelenskyy said in a message posted on Facebook.

The organizer of Ukrainian rallies in Melitopol, Olga Gaisumova was also captured and the city's aid headquarters in the Shevchenko recreation center was seized by Russian soldiers, who cordoned off the building, local blogger and activist, Tatiana Kumok, told ABC News in an interview.

The occupiers told the civil defense in Melitopol that they are prohibited from going in the streets during the curfew, according to Kumok.

Zelenskyy claimed the Ukrainians have had tremendous success, dealing "biggest blow to the Russian army in decades."

"We have had extraordinary success. How the entire Ukrainian people resisted these invaders has already gone down in history. But we have no right to reduce the intensity of defense, no matter how difficult it may be for us. We have no right to reduce the energy of our resistance," Zelenskyy said.

In his latest address, Ukrainian President Volodymyr Zelenskyy demanded the release of a mayor reportedly kidnapped Friday by Russian forces.

"Today in Melitopol, the invaders captured mayor of the city, Ivan Fedorov -- a mayor who courageously defends Ukraine and the people of his community," Zelenskyy said. "Obviously, this is a sign of the weakness of the invaders."

He called the alleged kidnapping a "crime against democracy" and said Russia's actions "will be equated with the actions of ISIS terrorists."

"Ukraine demands the immediate release of the mayor of Melitopol and guarantees of full security to all heads of communities across the country," he said, adding that he planned to raise this in talks with "international mediators who communicate with Moscow."

The southern Ukrainian city of Melitopol has been under Russian occupation since the first days of the invasion, though Fedorov had insisted it remained part of Ukraine.

Zelenskyy adviser Kirill Timoshenko posted a CCTV video Friday allegedly showing Fedorov being led out of the building by a large group of armed Russian soldiers.

The Treasury Department has announced new sanctions against Russia, targeting bankers, state officials, oligarchs and the family of Kremlin spokesman Dmitry Peskov.

Peskov was already sanctioned last week, but his wife and two adult children are now designated, with the Treasury saying they "live luxurious lifestyles that are incongruous with Peskov's civil servant salary and are likely built on the ill-gotten wealth of Peskov’s connections to Putin."

The U.S. is also sanctioning Russian oligarch and billionaire Viktor Vekselberg, going after assets that include a private jet and a yacht estimated to be a combined $180 million in value.

Others hit with sanctions include 10 people comprising VTB Bank's board, Russia's second largest bank, and 12 members of the State Duma.

-ABC News' Justin Gomez

The State Department is warning Americans of the risks of traveling to Ukraine to join in the fight against Russian troops.

"U.S. citizens should be aware that Russia has stated that it intends to treat foreign fighters in Ukraine as 'mercenaries,' rather than lawful combatants or prisoners of war," State Department spokesperson Ned Price said during a briefing Friday. "While we expect Russia to respect all of its obligations under the law of war, in light of this very concerning statement, U.S. citizens detained by Russian authorities in Ukraine -- they may be subject to potential attempts at criminal prosecution and may be at heightened risk for mistreatment."

When asked whether the U.S. Department of Justice would prosecute these Americans, Price said he believed they would not, but referred questions to the DOJ.

He declined to offer any metric for how many U.S. citizens have traveled to Ukraine, saying the State Department doesn't track Americans' travel.

ABC News reported Sunday that the Ukrainian embassy has received some 3,000 U.S. applications. Its defense attaché told The Associated Press Thursday that about 100 U.S. citizens have made the cut so far.

Secretary of State Antony Blinken had previously urged Americans to donate to aid groups or Ukrainian institutions, rather than travel to the country to take up arms.

-ABC News' Cindy Smith, Dee Carden and Conor Finnegan