Joint Chiefs Chairman General Mark Milley spoke to General Valery Gerasimov, the Russian Armed Forces chief of staff, and Ukraine armed forces Commander in Chief Ruslan Khomchak, the official said.

Russia's Ministry of Defense also confirmed the call, which came in a period of heightened tensions along Ukraine's long border with Russia.

On Tuesday each country accused the other of responsibility for an increase in violence between Ukraine government forces and Kremlin-backed separatists in eastern Ukraine, leading to a rise in deaths.

Khomchak denounced the "threat to the military security" of Ukraine by the Russian army, saying that some 28,000 separatist fighters and "more than 2,000 Russian military instructors and advisers" are currently stationed in eastern Ukraine.

Reports online suggested that Russia had located more military forces along the border and also moved more into Crimea, the Ukraine territory that Moscow seized in 2014.

Those reports could not be confirmed, but some observers have tied them to Russian military exercises.

The New York Times reported Wednesday that US forces in Europe were placed on a heightened watch level to "potential imminent crisis" in response to Russian activities.