Ukrainian President Volodymyr Zelenskyy has accused Russian forces of planning to stage "a fake rally in support of Russia" in the strategic port city of Kherson.

In a televised address Friday morning, Zelenskyy claimed that Russian troops were busing in "outsiders" from Moscow-annexed Crimea and "trying to recruit traitors from among the locals" to demand that Kherson be a Russian city.

"I appeal to the residents of Kherson: You can stop that, show them that Kherson is your city," Zelenskyy said. "We will not let go of what is ours."

"Show them our flags, sing our anthem, show your spirit, let them know that they can only stay in Kherson temporarily and would never be able to claim ownership of Kherson or any other city of our country," he added.

Russian forces took control of Kherson in southern Ukraine on Wednesday night. The Ukrainian government said earlier Friday that Russian troops have taken over Kherson's television tower and are broadcasting Russian channels, suggesting that Moscow may be planning to permanently occupy the city.

-ABC News' Patrick Reevell

Russia has selected and prepared a puppet government to install in Kyiv once its forces seize the Ukrainian capital, a senior U.S. administration official told ABC News.

U.S. intelligence believes Russian troops will ultimately crush Kyiv and decapitate its government, amid mounting evidence of indiscriminate shelling and a barrage against civilian targets across Ukraine, according to the official.

The official expressed concern that Ukraine lacks air power and what air force they had has been attacked, allowing Russia to mass its forces en route to Kyiv.

-ABC News' Martha Raddatz

Russian forces are advancing on Mykolaiv, another key city in southern Ukraine, the regional governor warned Friday.

In a video message posted on social media, Mykolaiv Oblast Gov. Vitaliy Kim said Russian troops are moving on Mykolaiv city from two directions and that some have already entered the city limits but are not yet inside in significant numbers.

The city is preparing to defend itself, according to Kim.

"Don't panic," Kim said. "At the moment, the enemy is approaching from two directions but they're not on our streets yet. We're preparing the defenses, so women and children should get home now and the men join the defense lines."

Kim said the Ukrainian military has a large amount of armour in Mykolaiv and urged residents "not to shoot at every vehicle," since some could be Ukrainian.

"No need to shoot at everything that's moving in the city. There's a lot of our armor in the city," he said. "{lease do not shoot inside the city, there's no enemy here yet, but they are approaching."

-ABC News' Patrick Reevell

The U.S. Embassy in Kyiv took to Twitter on Friday to condemn Russia's shelling of Ukraine's Zaporizhzhia nuclear power plant, the largest in Europe.

"It is a war crime to attack a nuclear power plant," the embassy tweeted. "Putin's shelling of Europe's largest nuclear plant takes his reign of terror one step further."

The United Nations High Commissioner for Human Rights has "received credible reports of several cases of Russian forces using cluster munitions, including in populated areas," spokesperson Elizabeth Throssell said Friday.

"Due to their wide-area effects, the use of cluster munitions in populated areas is incompatible with international humanitarian law principles," Throssell said.

Throssell added, "We remind Russian authorities that directing attacks against civilians and civilian objects as well as so-called bombardment in towns and villages and other forms of indiscriminate attacks are prohibited under international law and may amount to war crimes."

To date, there have been 26 attacks on health care facilities in Ukraine, killing at least 12 people and injuring 34 people, according to Jašarević. Two of those killed and eight of the injured were healthcare workers.

That number is "shocking," said Throssell.

Throssell and WHO spokesperson Tarik Jašarević declined to pin the blame for all of them on Russia.

This number of attacks includes Wednesday's strike on a children's hospital and maternity ward in Mariupol. On Thursday, Russian officials claimed that the attack was staged, but they first confirmed they bombed it and claimed the hospital was being used by Ukrainian "radicals."

Throssell told reporters that is not true; "It was a functioning hospital," she said.

-ABC News' Conor Finnegan