Russia on May 20 condemned NATO's invitation to Montenegro to become its 29th member, saying the step risked fuelling tensions in Europe. NATO's move on May 19 angered Russia, which feels threatened by a NATO expansion right up to its borders.
International Business Times: Russia condemns NATO’s membership invitation to Montenegro, says move could risk fuelling tensions

This picture shows a general view of the NATO Foreign ministers meeting at the NATO headquarters in Brussels on May 20.