More than 2.5 million people have fled Ukraine, UN agencies said on Friday, and a further 2 million have been driven from their homes within the country since the start of a Russian invasion on February 24.

The United Nations has been planning its humanitarian needs on the assumption that some 4 million Ukrainian refugees would seek safety abroad. However, with around 200,000 people having fled to neighbouring countries over the past 24 hours, a UN refugee official said they may have to revise that figure higher.

"It is quite possible that planning figure of 4 million will be revised up. That wouldn't be a surprise," Matthew Saltmarsh, a spokesperson with the UN refugee agency (UNHCR) near the border in Poland told Geneva journalists via videolink.

Humanitarian agencies are scrambling to provide heating facilities for thousands of refugees waiting in freezing temperatures at border crossings, he said. Meanwhile, trucks are taking thousands of thermal blankets and mattresses in the other direction.

Hundreds of thousands of people are trapped under heavy bombardment of cities and towns in Ukraine and are running out of supplies, with both sides blaming the other for failing to observe ceasefires. A report from the UN Office for the Coordination of Humanitarian Affairs said on Friday that psychological support services were being provided online from basements in Ukraine due to the security situation.