President Joe Biden alleged that Russia is intentionally targeting Ukrainian civilians.

But despite his accusations for the regime, Biden stopped short of formally claiming Moscow is committing war crimes.

His envoy to the United Nations, however, said Rssia is prueparing to use banned weapons, including "cluster munitions and vacuum bombs," in Ukraine.

She also issued a stark warning to invading Russian soldiers."Your leaders are lying to you. Do not commit war crimes," US Ambassador to the UN Linda Thomas-Greenfield said during remarks to an emergency session of the General Assembly. "Do everything you can to put down your weapons and leave Ukraine."

Russia's relentless bombardment of residential areas across Ukraine has already forced more than a million refugees to flee the country.

"I have worked in refugee emergencies for almost 40 years, and rarely have I seen an exodus as rapid as this one," Filippo Grandi, the UN High Commissioner for Refugees, said in a statement.

For comparison, it took three months for one million refugees to leave Syria in 2013.

Millions more people are likely to be forced to flee Ukraine, Grandi added. "International solidarity has been heartwarming. But nothing -- nothing -- can replace the need for the guns to be silenced; for dialogue and diplomacy to succeed. Peace is the only way to halt this tragedy."

In Ukraine's capital Kyiv, residents were awoken in the early hours of Thursday by at least one large explosion in the southwest of the city, following a day of heavy shelling.

To the south, the mayor of the strategically important city of Kherson on the Black Sea indicated that Russian forces had seized control, though claims remain disputed.

And in the port city of Mariupol, home to roughly 400,000, residents are without electricity and water, as Russian troops step up their offensive.

Ukrainian resistance in Kharkiv, the country's second-biggest city, continues to hold out, as Russian strikes hit at least three schools and damaged a cathedral and shops on Wednesday, according to videos and photos posted to social media, geolocated and verified by CNN.

One the same day, President Volodymyr Zelensky addressed Russian forces in a broadcast, saying: "You are leaving people without food, you are leaving us without medication. You are shelling evacuation routes, there's no weapon you wouldn't use against us, against free citizens of Ukraine."

According to Michelle Bachelet, UN High Commissioner for Human Rights, in addition to those who had left the country, one million people were also displaced internally.

"Thousands of people, including older people, pregnant women, as well as children and people with disabilities, are being forced to gather in underground shelters and subway stations to escape explosions," Bachelet said. "Many people in situations of vulnerability are separated from families and effectively trapped."

With the scale of the humanitarian disaster becoming ever more apparent, the International Criminal Court announced Wednesday it had opened an immediate active investigation into possible war crimes committed by Russian forces during the invasion of Ukraine.

ICC Prosecutor Karim A.A. Khan said his office "had already found a reasonable basis to believe crimes within the jurisdiction of the Court had been committed and had identified potential cases that would be admissible."

Let's hope that people responsible for these atrocities will be brought to justice.