Both Montenegro and Macedonia recognized Kosovo's independence on Thursday, despite opposition from Serbia, which called the moves by its Balkan neighbors a betrayal and expelled the Montenegrin ambassador from Belgrade.
The moves represent a major blow to Serbia's diplomatic efforts to maintain a claim over Kosovo, considered by Serbs to be the cradle of their Orthodox Christian religion and statehood.
Montenegro and Macedonia — both seeking membership in NATO and the European Union — have been under pressure from the United States and some EU countries to recognize Kosovo's February declaration of independence. The two coordinated with one another in recognizing Kosovo on Thursday, Montenegro's Foreign Minister Milan Rocen said.
"This is not a decision against Serbia, but for our future," Rocen said.
The Macedonian foreign minister also suggested the move was inevitable.
"This is a move that corresponds with reality," Antonio Milososki said. "We believe this (action) will not endanger our relations with Serbia."
Serbia had threatened unspecified retaliatory measures if the two countries recognized Kosovo, which Serbian Foreign Minister Vuk Jeremic said earlier would be "a stab in the back."
After Montenegro's recognition on Thursday, he said the country's ambassador "was no longer welcome" in Belgrade.
It was immediately unclear if Serbia would expel any diplomats from Macedonia, which announced its recognition of Kosovo several hours after Montenegro.
Jeremic said Montenegro's move was a particular betrayal, given that the country was both a close ally and a neighbor. Montenegro split from Serbia only in 2006, and its officials sided with Serbia during the region's ethnic wars in the 1990s. However, pro-Serb officials are not a part of the current government in Montenegro, a tiny Adriatic Sea state almost equally split between Montenegrin and pro-Serb nationalists.
Jeremic said the recognition jeopardized regional stability and amplified fears that tensions could be reignited.
"Regional countries have special responsibility in preserving peace and stability in the Balkans," Jeremic told the state-run Tanjug news agency.
Macedonia, which gained independence in 1991, was the 50th U.N. member country to recognize Kosovo. The recognition had been complicated by a border dispute, but the two sides say they are close to reaching a deal.
Meanwhile, Serbia won a diplomatic struggle Wednesday when the U.N. General Assembly agreed to ask the International Court of Justice for an opinion on the legality of Kosovo's declaration of independence.
Serbia said Thursday it was reinstating its ambassadors to the United States and countries including Japan, Canada and Australia that had recognized Kosovo's independence earlier this year. Belgrade had already returned its ambassadors to 22 EU nations in July.
Serbia said it was reinstating the diplomats due to "continued diplomatic activity to preserve its territorial integrity and sovereignty."
Serbia has not had effective control over Kosovo since 1999, when NATO led air strikes that halted former Serbian leader Slobodan Milosevic's crackdown on ethnic Albanian separatists. For years, it was administered by both NATO and the United Nations.