A satellite image shows Russian forces deployed in the western outskirts of Belgorod, Russia February 24, 2022. (Courtesy of Satellite image 2022 Maxar Technologies/Handout via REUTERS)

Ukrainian President Volodymyr Zelenskiy ordered a general mobilization, to be carried out within 90 days. 

Russian President Vladimir Putin said he did not plan a military occupation, only to disarm Ukraine.

Russian forces have reportedly captured the Chernobyl former nuclear power plant, just 90 km north of the capital Kyiv.

There was also fighting at Hostomel airport, just outside Kyiv, and in the regions of Sumy and Kharkiv in the northeast and Kherson in the south.

The UN refugee agency said an estimated 100,000 Ukrainians had fled their homes.  Some 57 people were killed, Ukraine's health minister said, while the interior ministry said 13 border guards died.

The US has ruled out sending troops to defend Ukraine, but Washington has reinforced its NATO allies in the region with extra troops and planes.

After consulting with the Group of Seven leading industrialised nations, US President Joe Biden announced measures to impede Russia's ability to do business in the world's major currencies, along with sanctions against banks and state-owned enterprises.

Britain also targeted banks, as well as members of Putin's inner circle. European Union leaders said measures would include freezing Russian assets in the 27-nation bloc.