Refugee agencies fear up to five million of Ukrainians could try to flee across the border as Russia's war intensifies and casualties mount.
Tens of thousands of people have already crossed into five countries bordering western Ukraine.
Poland alone has reported more than 50,000 arrivals in two days, while thousands more have streamed further south into Moldova, Romania, Hungary and Slovakia.
Most of those fleeing the invasion are women and children, as all men aged 18 to 60 are being told to stay and fight. BBC correspondents met them at the borders.
Source:
BBC