President Donald Trump approved Montenegro's accession to NATO on April 11, AFP reported. NATO foreign ministers formally invited Montenegro to start talks on joining the 28-member military alliance in December 2015. Russia has repeatedly warned against the move, calling it a threat to stability in the Western Balkans and an initiative with real potential to bring about confrontation. The U.S. Senate recently approved Montenegro's bid, and the White House approval was the last major U.S. domestic hurdle. After Canada ratified the protocol in March, only Spain and the United States remained. Spain has said it plans to do so, but the measure has not yet passed fully through its parliament. Montenegro will also have to approve the decision to join NATO, and the opposition has been calling for a general referendum on the issue. NATO member states will meet in May for a summit, where representatives will need to approve Montenegro's membership. The U.S. president has harshly criticized NATO member states that fail to meet the NATO defense spending target of 2 percent of GDP. According to the World Bank, Montenegro spent 1.62 percent of GDP on its military.