Ukrainian President Volodymyr Zelenskyy says Russian forces are trying to seize the Chernobyl nuclear plant just 130 kilometres north of Kyiv.
The plant was the site of the world's worst nuclear accident in April 1986 and its exploded reactor has been covered by a protective shelter to prevent radiation leaks.
There are widespread fears Russia's invasion could cause massive casualties, topple Ukraine's democratically elected government and upend the post-Cold War security order.
As Russian forces advance on several fronts, Kyiv's mayor is urging the capital's three-million residents to stay indoors unless they work in critical sectors.
He says everyone should prepare go-bags with necessities such as medicine and documents.