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

Ukrainian technicians have started repairing damaged power lines in an attempt to restore external electricity supplies to the site of the Russian-controlled Chernobyl nuclear power plant that were entirely cut earlier in the week, according to International Atomic Energy Agency Director General Rafael Mariano Grossi.

Ukraine’s regulatory authority said work began Thursday and technicians had succeeded in repairing one section, but off-site electrical power was still down, indicating there was still damage in other places.

Emergency diesel generators have been providing backup power to the site since Wednesday, and the regulator has reported that additional fuel had been delivered to the facility, the IAEA said.

The regulator lost communications with the site on Thursday and, as a result, it cannot provide information to the IAEA about the radiological monitoring at the facility, according to Grossi.

The regulator is still receiving information about the situation there through senior off-site management of the plant, Grossi said.

The plant's disconnection from the grid will not have a critical impact on essential safety functions at the site, however, staff is facing increasingly difficult conditions.

At least 211 technical personnel and guards have been living at the site for more than two weeks, and there is concern about the availability of food reserves, the regulator said.

The IAEA said eight of the country’s 15 reactors remained operating. Radiation levels at the locations were normal, the IAEA said.

-ABC News' Zoe Magee

Russian forces have kidnapped the mayor of the southern Ukrainian city of Melitopol, which is currently under Russian occupation, according to a Ukrainian official.

Melitopol has been occupied since the first days of the invasion, but its mayor, Ivan Fedorov, had insisted it remained part of Ukraine. He was taken from his office by Russian troops Friday afternoon, according to Kirill Timoshenko, an advisor to Ukraine’s president.

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

Local people have been protesting the Russian occupation almost everyday, marching in the city with Ukrainian flags and confronting Russian soldiers.

A local woman who has taken part in the protests, Tatiana Kumok, in a phone call with ABC News, said Russian riot police surrounded the central square and tried to prevent protests last week.

Kumok said that local TV has been switched to Russian channels and that a new Russian "police force" has been deployed on the streets.

Kumok said she fears that Russia plans to permanently occupy Melitopol, even if Russia eventually reaches a deal with Kyiv. She said they have been told they will be given Russian passports.

-ABC News' Patrick Reevell

Ukraine claimed Thursday that residents of Mariupol who have survived Russian bombardment are now being forcibly deported to Russia.

"The Russian Federation has launched a new phase of terror against the city of Mariupol," the Ukrainian Ministry of Foreign Affairs said in a statement. "According to available information, the Russian army has forcibly deported about 6,000 Mariupol residents to Russian filtration camps in order to use them as hostages for political pressure on Ukraine."

"At the same time, the Russian armed forces are firing on evacuation columns trying to leave Mariupol for the unoccupied territory of Ukraine," the ministry added. "Russian troops continue to hold a humanitarian convoy of buses that arrived a few days ago from Mariupol from Zaporizhia."

According to the ministry, some 15,000 residents of the besieged port city in southeastern Ukraine are in danger of being forcibly deported to Russia, with Russian troops confiscating peoples' passports and other identification documents.

"Such actions by Russia are a gross violation of the laws or customs of war, the rules of international humanitarian law," the ministry said.

The ministry called on world leaders to "take urgent action to save the lives of residents of Mariupol and other Ukrainian cities who have been in an inhumane siege by the Russian army."

"The international community must impose new tough sanctions on Russia to stop its deadly military machine, as well as cut off all business ties with Russian companies to stop funding Russia's war against Ukraine," the ministry said.

-ABC News Julia Drozd