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.
More top news
The cult musical Rocky Horror Show returns to the London stage in September with its writer Richard O'Brien as the narrator.
George Orwell's Nineteen Eighty-Four is at the top of teachers' list of books "every student should read before leaving secondary school".
Film director Guy Richie marries his long-term partner Jacqui Ainsley at their country home in Wiltshire.