The Chernobyl nuclear power plant has been captured by Russian forces, an adviser to the Ukrainian presidential office, Mykhailo Podolyak, said on Thursday.

"It is impossible to say the Chernobyl nuclear power plant is safe after a totally pointless attack by the Russians," he said.

"This is one of the most serious threats in Europe today," Podolyak said.

Russian troops took over the power plant while Ukrainian forces battled them on three sides on Thursday after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since World War Two.