More than 874,000 people have fled Ukraine since Russia's invasion last week, the UN refugee agency says, adding that the figure is growing exponentially which could push it past the 1 million mark within hours, AP reported.

UNHCR spokeswoman Shabia Mantoo said people continue to flee to Ukraine's neighbors to the west, with more than 200,000 fleeing since Tuesday.

A day earlier, Mantoo had warned that an exodus of refugees from Ukraine could make it the source of the biggest refugee crisis this century.

She noted that the UNHCR had previously forecast that up to 4 million people could leave Ukraine, but said the agency would revise its forecast.

The latest figures show that more than half, or almost 454,000 people, went to Poland, more than 116,300 to Hungary and more than 79,300 to Moldova. Another 69,000 left for other European countries, while 67,000 fled to Slovakia.