Brussels: US Secretary of State Antony Blinken said Friday that President Vladimir Putin's decision to send reinforcements to Ukraine's Kharkiv region underlined the huge losses Russia's forces are taking in its war.

"There are a huge number of Russian forces that are in Ukraine and unfortunately, tragically, horrifically President Putin has demonstrated that he will throw a lot of people into this at huge cost to Russia, at huge cost to its future," he said.

Blinken was speaking to reporters at NATO headquarters in Brussels after talks with the Western alliance's Secretary General Jens Stoltenberg, one day after the chief US diplomat had visited Kyiv.

Earlier, Russian news agencies had reported that Moscow's forces were redeploying to the Kharkiv region in response to a large-scale Ukrainian counter-offensive.

The TASS and RIA Novosti news agencies shared video from Russia's defence ministry, showing the movement of military hardware on a paved road, some with Russian flags and bearing the letter "Z", a symbol of Russia's military campaign.

Local Moscow-installed official Vitaliy Ganchev said in televised remarks that "fierce battles" were under way near Balakliya, a town in Kharkiv region that Ukraine said it had recaptured on Thursday.