Montenegro was invited to join the military bloc in 2015. In May 2016, the Alliance members signed a protocol on Podgorica's accession, which should be ratified by all the NATO member states to allow Montenegro become a full-fledged member.
"Unfortunately, Montenegro has yielded to the ultimatum ‘either with NATO, or with Russia’ and decided to sacrifice relations with our country, damaging its economic interests… It is absolutely unclear how NATO's security will benefit from Montenegro's accession, and it is absolutely unclear how Montenegro’s security will benefit from it," Lavrov said.
Earlier in March, Russian envoy to NATO Alexander Grushko told Sputnik that Montenegro's dragging into NATO was an attempt to show that the open door policy was alive, however, many understand its erroneous nature.
In February, Russian President Vladimir Putin said that NATO used an alleged threat from Moscow as a justification for its expansion. Several countries are actively negotiating NATO's accession, including Georgia and Ukraine, among others.