The US State Department said it was told Russia has accumulated tanks and artillery at deployment sites that may be given to separatist fighters in Ukraine.
The US also has its own information that Russia redeployed military forces to its border with Ukraine, State Department spokeswoman Jen Psaki said.
"We have information that additional tanks have been prepared for departure," she added.
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The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.