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 Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler