The Kremlin on Thursday denounced US plans to provide depleted uranium tank rounds to Ukraine, saying Washington would be responsible for any negative lingering impact of the munitions' use.

Depleted uranium munitions are controversial due to their association with health problems such as cancer and birth defects in areas where they were used in past conflicts, though they have not been definitively proven to have caused such issues.

The Pentagon announced this week it would provide Ukraine with the rounds as part of a $1 billion aid package that coincided with an unannounced visit to Kyiv by US Secretary of State Antony Blinken.

"This is very bad news," Kremlin spokesman Dmitry Peskov said.

"The use of these shells has resulted in a galloping increase in the number of cancer patients ... The same situation will inevitably await those Ukrainian territories where they will be used."

"Responsibility will lie with the US," he added.

Russian President Vladimir Putin has warned that he would deploy depleted uranium ammunition if Ukraine received the arms.

Ukraine, which has been asking for more Western help, launched a counteroffensive against Russian forces in June after building up assault battalions and stockpiling Western weapons.

Peskov also criticised Blinken's announcement that the US would give Ukraine $5.4 million in forfeited oligarch assets to support veterans as part of the package.

"We consider all cases related to the seizure and other withholding of any funds related to state property or private property of the Russian Federation to be illegal," Peskov said.

"One way or another they will lead to legal proceedings."