The Pentagon placed 8,500 U.S. military personnel on heightened alert earlier this week for possible deployment, a move that came as NATO weighs a possible activation of its 40,000-strong response force to deter a Russian invasion.

Chair of the Joint Chiefs of Staff, Gen. Mark Milley told reporters at the Pentagon Friday that the troops have been authorized to "increase our readiness in the event we have to reinforce or assist our NATO allies."