The UN refugee agency, UNHCR, said on Wednesday that 874,026 people have fled Ukraine, more than half of which have fled to Poland, according to its count.

According to the UNHCR, 453,982 Ukrainians have fled to Poland, 116,348 people to Hungry and 79,315 to Moldova. The agency said 69,600 people have fled to other European nations.

The World Health Organization on Wednesday warned of a possible COVID-19 surge in Ukraine and critical oxygen shortages with at least three major oxygen plants in the country closed.

Ukraine is experiencing a critical shortage of oxygen impacting the ability to treat those patients.

WHO Director General Dr. Tedros Ghebreyesus said that just prior to the war, Ukraine was experiencing a surge in COVID-19 cases.

With low testing rates since the start of the conflict, there's likely to be significant undetected transmissions, Ghebreyesus said.

Ghebreyesus warned that with low testing and vaccination rates in Ukraine, the risk of large numbers of people developing severe disease increases.

The WHO said it is seeking ways of accessing oxygen from neighboring countries and ways to deliver it safely, saying there is an urgent need to establish a corridor for humanitarian workers and supplies to reach people in need.

Ghebreyesus said he is deeply concerned about the unfolding humanitarian emergency in Ukraine. The first WHO shipment meant to meet the needs of 1,000 patients in Ukraine will arrive in Poland on Thursday. It contains 6 metric tons of supplies for trauma care and emergency surgery.

The WHO has released $5.2 million from their contingency fund for emergencies and said it will need $45 million for Ukraine for the next 3 months and $12.5 million to support neighboring countries in their care for refugees.

-ABC News' Christine Theodorou

Nearly 2,870 Ukrainian nationalists and army have died and some 3,700 have been injured, according to the Russian Defense Ministry.

According to confirmed data from the Russian Defense ministry, 572 Ukrainian troops have been captured.

Russia said civilians wishing to leave Kyiv in the direction of Vasylkiv won't be stopped by its forces.

"Russian troops are not hindering the exit of the civilian population," Russian Defense Ministry spokesman Igor Konashenkov said.

The Russian Defense Ministry also said civilians can leave the city of Mariupol.

"I would like to emphasize that all civilians wishing to leave Mariupol for safety reasons can exit in the eastern direction via the Mariupol-Shyrokyne motorway," Konashenkov said.

-ABC News' Anastasia Bagaeva

Ukrainian President Volodymyr Zelenskyy on Wednesday praised the United Nations General Assembly for its vote to condemn the Russian invasion and call for the withdrawal of Russian troops.

Zelenskyy called the results of the vote "destructive" for the aggressor, and said it shows "a global anti-Putin coalition has been formed and is functioning."

“I praise the approval by the #UN GA with an unprecedented majority of votes of the resolution with a strong demand to Russia to immediately stop the treacherous attack on. I’m grateful to everyone & every state that voted in favor. You have chosen the right side of history," Zelenskyy said in a tweet.

"The world is with us. The truth is on our side. Victory will be ours!” Zelenskyy said.

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