Russia is feeding its people a "steady diet of lies" to conceal the true costs it is incurring as a result of its more than half-year war against Ukraine, the State Department said on Tuesday.

Spokesman Ned Price said Russian President Vladimir Putin and senior officials "have done everything they can" to limit the flow of unvarnished information to the Russian people and have ensured they are given a constant flow of government propaganda.

Putin and his allies "feed the Russian people a steady diet of lies, disinformation in an effort to hide the true costs of this war as the war has ground on, as additional Russians have come home in body bags, come home missing limbs," Price told reporters.

The comments come amid a lighting Ukrainian offensive that has led to mass territory being re-taken from Russian forces in the northeast, including the pivotal cities of Izyum and Kupiansk, both of which served as critical supply hubs for Moscow's forces in the region.

Price said there have been signs of greater opposition to the war fomenting in Russia, including what he said are "high-profile instances of defiance."

"There has also been a sort of lower level question and dialogue that you point to across Russia," he said.

"Unfortunately, this is a system that is ruled by and large by one individual. It is an individual surrounded not by democratically-elected aides, but by individuals who have set up, in many ways, safeguards against popular opinion, in some cases, even reaching the innermost sanctum of the Kremlin," he added.

Ukraine's recent rapid advances have seen it retake more than 3,000 square kilometers (1,158 square miles) of territory in and near the northeastern region of Kharkiv.