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

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

Pentagon spokesman John Kirby told reporters Friday that the U.S. doesn't have "firm indications" that Russia is planning a so-called "false flag" operation to justify use of chemical or biological weapons, but that it is something officials think "could happen that we want to watch out for."

"We're watching this as closely as we can," Kirby said during a briefing, adding that he didn't have anything specific to report on Russian chem-bio capabilities inside Ukraine.

"We continue to watch for the potential -- and I want to stress the word potential -- that they could be banging this drum with the intent of creating some sort of false flag event that they could use as an excuse to escalate the conflict even more," he continued. "I don't have any specific indication now to talk about, but it is something we're concerned about."

Kirby described how Russia possesses a biochemical weapons program and has a "reputation" for using such weapons in the past.

As for concerns that Russia was planning a false flag of a Ukrainian attack in Belarus Friday, Kirby said he had seen the claims by the Ukrainian Defense Ministry but had nothing to corroborate them.

Ukraine's air force claimed Friday that Russia did carry out an alleged false flag airstrike in a Belarusian village near the border with Ukraine.

-ABC News' Luis Martinez