Russia has amassed around 20,000 troops on Ukraine's eastern border and could use the excuse of an humanitarian or peacekeeping mission to send them into Ukraine, NATO said in a statement on Wednesday.
"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground - and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," NATO spokeswoman Oana Lungescu said in a statement.
"The latest Russian military build-up further escalates the situation and undermines efforts aimed at finding a diplomatic solution to the crisis. This is a dangerous situation."
Polish Prime Minister Donald Tusk said on Wednesday the threat of a direct intervention by Russia's military in Ukraine has risen over the last couple of days.
NATO's statement said it was concerned Moscow could use "the pretext of a humanitarian or peacekeeping mission as an excuse to send troops into Eastern Ukraine."
(Reporting by Barbara Lewis and Tom Koerkemeier; editing by Foo Yun Chee)