KYIV, July 27 (Reuters) - Russian forces have taken over Ukraine's second biggest power plant, an adviser to President Volodymyr Zelenskyy said on Wednesday (Jul 27), following an earlier claim by Russian-backed forces to have captured it intact.

Seizing the Soviet-era coal-fired Vuhlehirsk power plant in eastern Ukraine would be Moscow's first strategic gain in more than three weeks in what it calls its "special operation" to demilitarise and "denazify" its neighbour.

"They achieved a tiny tactical advantage - they captured Vuhlehirsk," adviser Oleksiy Arestovych said in an interview posted on YouTube.

Russia was undertaking a "massive redeployment" of troops to three southern regions, Arestovych said.

US Secretary of State Antony Blinken, whose government describes Russia's invasion as an unprovoked war of aggression, said on Wednesday that he planned a phone conversation with Russian foreign minister Sergei Lavrov - the first between the two diplomats since before the start of the war.

The call would not be "a negotiation about Ukraine," Blinken said at a news conference, restating Washington's position that any talks on ending the war must be between Kyiv and Moscow.

Russia has received no formal request from Washington about a phone call between Blinken and Lavrov, TASS news agency reported.

The United States has made "a substantial offer" to Russia for it to release US citizens WNBA star Brittney Griner and former US Marine Paul Whelan, Blinken said, without providing details.

Aside from discussing Americans detained by Russia, Blinken said he would raise with Lavrov the tentative deal on grain exports reached last week between Russia, the United States, Turkey and Ukraine.

Rising energy prices and a global wheat shortage that threatens millions in poorer countries with hunger are among the far-reaching effects of the war.