Evacuations of people fleeing embattled Ukrainian cities along safe corridors began Tuesday, while U.N. officials said the exodus of refugees from Russia’s invasion reached 2 million.

The Russian onslaught has trapped people inside cities that are running low on food, water and medicine amid the biggest ground war in Europe since World War II.

Previous attempts to lead civilians to safety have crumbled with renewed attacks. But on Tuesday, the video posted by Ukrainian officials showed buses with people moving along a snowy road from the eastern city of Sumy and yellow buses with a red cross on them heading toward the southern port of Mariupol.

It was not clear how long the efforts would last.

“The Ukrainian city of Sumy was given a green corridor, the first stage of evacuation began,” the Ukrainian state communications agency tweeted.

While some people fled to other cities in Ukraine, many have chosen to leave the country instead. Safa Msehli, a spokesperson for the U.N.’s International Organization for Migration, tweeted that 2 million have now left, including at least 100,000 people who are not Ukrainian.