LONDON, Jan 20 (Reuters) - British Prime Minster Boris Johnson spoke with German Chancellor Olaf Scholz on Thursday, discussing their concern over Russia"s actions towards Ukraine, and with Britain stressing the need for NATO allies to act together in response. "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."