Ukrainian President Volodymyr Zelenskyy issued a warning to his Russian President Vladimir Putin on Thursday, as Russian forces continued their offensive.

"You will repay everything you did against Ukraine -- in full," Zelenskyy said in a televised statement. "And we will not forget those who perished -- and God won't."

Zelenskyy noted that his country receives "more and more powerful weapons everyday" from "partners" and "real friends." He also likened the Russian invasion to being "attacked by another virus."

"Exactly two years ago, the first case of COVID-19 was recorded in Ukraine. The first weeks of fighting it were extremely difficult, but we were united and therefore strong and therefore we withstood," he said. "Exactly a week ago, Ukraine was attacked by another virus, another disease, by those who suffer from severe annexation and occupation of foreign lands. The first hours and days of full-scale war were extremely difficult, but we were united and therefore strong and therefore we withstood. And it will be so and we will continue to stand."

Russia's state space corporation Roscosmos announced Thursday its decision to suspend supplies of Russian-made rocket engines to the United States.

"In this situation, we can no longer supply the U.S. with our rocket engines that are the best in the world," Roscosmos CEO Dmitry Rogozin said on state-owned television channel Russia-24. "Let them fly on something else, like their brooms or whatever. But at least we are freezing our shipments."

-ABC News' Tanya Stukalova

Russian forces continue to intensely bombard the key Ukrainian port city of Mariupol, according to local councillor Petro Andrushenko.

Speaking to ABC News by telephone from a bomb shelter in Mariupol on Thursday, Andrushenko said Russian forces have been striking the southern city with missiles and heavy artillery non-stop for more than 24 hours. He said the firing was continuing even as he spoke, hitting the city center.

Mariupol is now besieged and surrounded by Russian troops. A last column of journalists and diplomats managed to pull out on Wednesday under the guns of advancing Russian forces.

Andrushenko said one neighborhood, Livoberezhna, has been "destroyed" and that authorities have tried to evacuate the residents there. The entire city is without power and has waning supplies, according to Andrushenko.

"We haven't any heat, we haven't any water, we haven't any electricity, but we have Russian rockets," he told ABC News.

At least 10 people have been killed and 150 others have been injured in Mariupol so far, according to Andrushenko. But it's virtually impossible to get an accurate count because authorities are unable to recover bodies under such heavy bombardment.

Andrushenko said he believes Russia is trying to make Mariupol "like Aleppo," the Syrian city that the Russian military helped Syrian government forces devastate during a siege there in 2016. Aleppo ultimately became a symbol of the brutality of the Syrian civil war.

“They want to do like Aleppo for Mariupol now," Andrushenko said, "because Mariupol is a symbol of Ukrainian power."

-ABC News' Patrick Reevell

More than 1 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations High Commissioner for Refugees (UNHCR).

Over 50% of the refugees from Ukraine are in neighboring Poland, UNHCR figures show.

"In just seven days, 1 million people have fled Ukraine, uprooted by this senseless war," U.N. High Commissioner for Refugees Filippo Grandi said in a statement Thursday. "I have worked in refugee emergencies for almost 40 years and rarely have I seen an exodus as rapid as this one."

"Hour by hour, minute by minute, more people are fleeing the terrifying reality of violence. Countless have been displaced inside the country," he added. "Peace is the only way to halt this tragedy."

-ABC News' Zoe Magee

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