The UN said Tuesday that two million people have fled Ukraine since the start of Russia's invasion.

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The UN said one million children have left Ukraine.

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James Elder, a spokesman for Unicef, described 1 million children fleeing as a dark historical first, and it means children make up just under half of the 2 million people that have fled Ukraine since February 24.

More than half of refugees have fled to neighboring Poland, with many also fleeing to Hungary, Slovakia,Moldova, and Romania.

Some 210,000 refugees have fled to other European countries.

A significant portion of refugees have traveled to Russia, with a small number also going to its ally, Belarus.

Elder lamented the scale of the refugee crisis in an interview with CNN on Monday. Russian attacks on key civilian facilities have left millions without water and critical infrastructure. Immediate cessation of hostilities is the fastest way to resolve the crisis.

Big Number

There are 4 million. The UN expects a lot of people to flee Ukraine due to the conflict. European officials think the figure could go as high as 7 million, with 18 million Ukrainians affected in humanitarian terms.

Key Background

The biggest refugee crisis in Europe since World War Two has been caused by Russia's invasion of Ukraine. Many people are likely to be displaced within Ukraine. The humanitarian crisis in Ukraine has been worsened by the bombardment of cities by Russian forces and attempts to establish safe escape routes for civilians fleeing have failed. Russian forces were said to have violated a ceasefire and prevented civilians from leaving as they continued to shell the area, and Ukraine later rejected the offer of humanitarian corridors that would only allow Ukrainians to evacuate to Russia. A new humanitarian corridor opened up on Tuesday allowing residents of the Ukrainian city of Sumy to travel to other parts of the country.

Humanitarian corridors are what you need to know as refugees flee.