Washington DC – Russia is executing soldiers who don't follow orders in its bloody new offensive in eastern Ukraine while suffering "significant" losses of armor and personnel, the White House said Thursday.

"We have information that the Russian military has been actually executing soldiers who are refusing to follow orders," National Security Council spokesperson John Kirby told a briefing.

"We also have information that Russian commanders are threatening to execute entire units if they seek to retreat from Ukrainian artillery fire," he added.

"It's reprehensible."

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Kirby did not give further details when asked about the alleged executions or how the US had obtained the information.

Russia has stepped up attacks near the town of Avdiivka in Ukraine's eastern Donetsk region and the northeastern city of Kupyansk in a bid to move the front line forward before winter.

Moscow has suffered "significant losses" in the offensive including at least 125 armored vehicles and "thousands" of casualties, Kirby said.

"The Russian military appears to be using what we would call human wave tactics," he said.

"It is unsurprising that Russian forces are suffering from poor morale," Kirby added.