Ukraine said it would not accept ultimatums from Russia after Moscow demanded it stop defending besieged Mariupol, where hundreds of thousands of civilians are suffering through Russian bombardments laying waste to their city.

Mariupol has become a focal point of Russia's assault on Ukraine, but attacks were also reported to have intensified on the country's second city Kharkiv on Monday.

The conflict has driven almost a quarter of Ukraine's 44 million people from their homes, and Germany predicted the refugee number could reach as high as 10 million in coming weeks.