In an exclusive interview with ABC News anchor David Muir, Ukraine President Volodymyr Zelenskyy stressed that the United States and NATO should establish a no-fly zone over his country to stop Russian missiles from hitting civilian targets like a university dormitory and a pediatric clinic.

"Mr. President, as you know, the president [Joe Biden] and NATO have said no to this no-fly zone because of concerns this could trigger a much wider ... conflict, a much bigger war than what we're seeing already because there would have to be a willingness to shoot Russian planes outta the sky. Do you understand that concern?" Muir asked.

Zelenskyy, speaking to Muir from the Presidential Office in Kyiv, said that while Biden and NATO have refused to impose a no-fly zone, Russian missiles continue to target civilians, adding that he believes Russian missiles should be shot out of the sky.

"You have to preserve lives. There ... were simply kids there with tumors," Zelenskyy said of the missile strike on the pediatric clinic. "And in the university, there were ordinary students. I'm sure that the brave American soldiers who would be shooting it down knowing that it is flying towards the students, I'm sure that they had no doubt in doing so."

The White House indicated on Monday that it was considering a ban on Russian oil even without the support of its European allies, an official with the National Security Council told ABC News.

Europe relies much more on Russian oil and gas than the United States does and would likely see much higher prices if such a significant chunk of their energy imports were suddenly cut off.

If the United States banned Russian oil imports without Europe doing the same, the impact on Russia would be much smaller than if the U.S. and Europe worked in concert. Russia exported 1% of its oil exports to the United States in 2020.

Moving forward without Europe would also be a departure from Biden’s approach of working in lockstep with Europe on sanctions against Russia.

Bloomberg reported Sunday night that the Biden administration was weighing this move.

-ABC News' Ben Gittleson

In an exclusive interview Monday with ABC News anchor David Muir, Ukraine President Volodymyr Zelenskyy said the path to peace is "more difficult than simply acknowledging" another ultimatum from Russian President Vladimir Putin.

Zelenskyy spoke with Muir from the Presidential Office in Kyiv and said he will stay in his country for the duration of the war.

"When the Kremlin says these three conditions to end the war … that you must give up on joining NATO, recognize Crimea as part of Russia and recognize the independence of those two separatist regions in the east … you say it’s a nonstarter, not willing to [accept] those three conditions right now?" Muir asked.

Zelenskyy responded, "It is more difficult than simply acknowledging them ... this is another ultimatum and we are not prepared for ultimatums."

He went on, "But we have the possible solution resolution for these three items, key items, what needs to be done is for President Putin to start talking. Start the dialogue instead of living in the information bubble without oxygen. I think that’s where he is. He is in this bubble. He’s getting this information and you don’t know how realistic that information is that he’s getting."

President Joe Biden is scheduled to hold a secure video teleconference Monday morning with leaders of France, Germany and the United Kingdom to discuss the latest developments regarding Russia and Ukraine, according to the White House.

Biden is expected to be in the White House Situation Room when he participates in a teleconference with French President Emmanuel Macron, German Chancellor Olaf Scholz and British Prime Minister Boris Johnson, according to the White House.

The teleconference will be closed to the press and will occur after Biden received his daily intelligence briefing, the White House said.

-ABC News' Ben Gittleson

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