Out of the more than 2 million people who have been forced to flee Ukraine since Russian forces invaded on Feb. 24, half of them are children, according to the United Nations International Children's Emergency Fund (UNICEF).

UNICEF spokesperson James Elder called it a "dark historical first."

Energy giant Shell announced Tuesday plans to withdraw from its involvement in all Russian hydrocarbons, including crude oil and natural gas, amid Russia's unprovoked invasion of neighboring Ukraine.

"As an immediate first step, the company will stop all spot purchases of Russian crude oil. It will also shut its service stations, aviation fuels and lubricants operations in Russia," Shell said in a statement.

Shell will immediately stop buying Russian crude oil on the spot market and not renew term contracts. The company will also change its crude oil supply chain to remove Russian volumes, but said "this could take weeks to complete and will lead to reduced throughput at some of our refineries."

In addition, Shell will shut its service stations, aviation fuels and lubricants operations in Russia, and will start a phased withdrawal from Russian petroleum products, pipeline gas and liquefied natural gas.

The company apologized for buying Russian oil last week.

"We are acutely aware that our decision last week to purchase a cargo of Russian crude oil to be refined into products like petrol and diesel -- despite being made with security of supplies at the forefront of our thinking -- was not the right one and we are sorry," Shell CEO Ben van Beurden said in a statement. "As we have already said, we will commit profits from the limited, remaining amounts of Russian oil we will process to a dedicated fund. We will work with aid partners and humanitarian agencies over the coming days and weeks to determine where the monies from this fund are best placed to alleviate the terrible consequences that this war is having on the people of Ukraine."

Ukrainian Foreign Minister Dmytro Kuleba took to Twitter to praise Shell "for taking this moral and responsible step," and he encouraged "other world businesses to follow suit."

At least 21 civilians, including two children, were killed by a Russian airstrike in Ukraine's northeastern city of Sumy on Monday night, according to the regional prosecutor's office.

The strike hit a residential area of Sumy, according to the State Emergency Service of Ukraine, which the regional prosecutor's office said was still on the scene searching for victims Tuesday.

Ukrainian Deputy Prime Minister Iryna Vereshchuk called on Russian forces to maintain the agreed upon temporary cease-fire in Sumy and four other Ukrainian cities to allow civilians to evacuate Tuesday. She said Russian authorities have confirmed to the International Committee of the Red Cross that one evacuation route out of Sumy will be open, but Ukrainian officials are awaiting confirmation on the other routes they submitted.

-ABC News' Julia Drozd and Patrick Reevell

More than 2 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 1.2 million of the refugees from Ukraine are in neighboring Poland, UNHCR figures show.

"Today the outflow of refugees from Ukraine reaches two million people. Two million," U.N. High Commissioner for Refugees Filippo Grandi said Tuesday in a post on his official Twitter account.

-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