Russia's buildup of military personnel threatening Ukraine probably totals up to 190,000, the U.S. ambassador to the Organization for Security and Cooperation in Europe said in a statement to an OSCE meeting on the Ukraine crisis on Friday.

"We assess that Russia probably has massed between 169,000-190,000 personnel in and near Ukraine as compared with about 100,000 on January 30," Michael Carpenter told the meeting, which Russia did not attend.

"This is the most significant military mobilization in Europe since the Second World War."