"The leaders discussed the concerning developments on the Ukrainian border, and agreed further military aggression would come at a high cost for Russia," Johnson's office said in a statement

"They shared their deep concern at the ongoing destabilising action by Russia in Ukraine, and said any invasion into Ukraine would be a severe strategic mistake.

"The Prime Minister stressed the importance of NATO allies working together on a coordinated response."

(Reporting by William James,Editing by Franklin Paul)