RUSSIAN forces have taken over Ukraine's second-biggest power plant in eastern Donetsk region and are undertaking a "massive redeployment" of troops to three southern regions, a Ukrainian presidential adviser said.

Russian-backed forces claimed on Wednesday that they had captured the Soviet-era coal-fired Vuhlehirsk power plant intact, in what was Moscow's first strategic gain in more than three weeks.

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

Arestovych said that the Russian redeployment appeared to be changing tactics to strategic defence from offence in what Moscow calls its "special operation" to demilitarise and "denazify" its neighbour. Russia invaded Ukraine on Feb. 24

Arestovych and another Ukrainian official said Russia was sending troops to the Melitopol and Zaporizhzhia regions and Kherson. Ukraine has shelled an important bridge straddling the Dnipro river in Kherson, closing it to traffic. Russian officials had earlier said they would turn instead to pontoon bridges and ferries to get forces across the river.

In a Wednesday evening address, Zelenskiy said Ukraine would rebuild the Antonivskyi bridge over the Dnipro and other crossings in the region.

"We are doing everything to ensure that the occupying forces do not have any logistical opportunities in our country."

In Washington, U.S. Secretary of State Antony Blinken, whose government describes Russia's invasion as an unprovoked war of aggression, said 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 in the coming days 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 U.S. citizens WNBA star Brittney Griner and former U.S. Marine Paul Whelan, Blinken said, without giving details of what the United States was offering in return.

Blinken said he would press Lavrov to respond to the offer.

A source familiar with the situation confirmed a CNN report that Washington was willing to exchange Russian arms trafficker Viktor Bout, who is serving a 25 year-prison sentence in the United States, as part of a deal.

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.

GAS AND GRAINS

Russia reduced gas flows to Europe on Wednesday in an energy stand-off with the European Union. It has blocked grain exports from Ukraine since invading, but on Friday agreed to allow deliveries through the Black Sea to Turkey's Bosphorus Strait and on to global markets. 

The deal was almost immediately thrown into doubt when Russia fired cruise missiles at Odesa, Ukraine's largest port, on Saturday, just 12 hours after the deal was signed.

Before the invasion and subsequent sanctions, Russia and Ukraine accounted for nearly a third of global wheat exports.

Russian and Russian-backed forces have been struggling to make meaningful progress on the ground since their capture in early July of the eastern Ukrainian city of Lysychansk.