Russian forces have seized control of a key port city in southern Ukraine, the mayor says.

Kherson is the first major city to be taken by Russia, after heavy fighting, since it invaded a week ago.

Its mayor, Igor Kolykhaev, said Russian troops had forced their way into the city council building and imposed a curfew on residents.

Several cities have come under intense shelling, with Wednesday one of the most destructive days of the fighting.

In a Facebook post, Mr Kolykhaev said Russian forces were in control of Kherson, a port on Ukraine's southern Black Sea coast with a population of more than 280,000 people.

He urged Russian soldiers not to shoot at civilians, saying there were no Ukrainian forces in the city.