KYIV, March 2 (BBC) - 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.

An investigation into possible war crimes and crimes against humanity committed in Ukraine has been launched by the International Criminal Court (ICC) in The Hague.

Russia has for the first time admitted taking heavy military casualties during its attack on Ukraine, with 498 troops killed and a further 1,597 injured. Ukraine says Russia's losses run into the thousands.

Ukraine reports that more than 2,000 civilians have died since the invasion began last Thursday. The conflict has also caused 1,000,000 people to flee Ukraine, the UN's High Commissioner for Refugees Filippo Grandi said.

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.