Lawmakers in Kosovo have passed legislation to build a full-fledged army, a move that has inflamed tensions with its former wartime foe Serbia.
The United States said the move was "only natural" for a "sovereign, independent country" and urged continued dialogue with Serbia.
However, NATO called the decision "ill-timed" and said it went against the advice of the alliance.
Russia, Serbia's traditional ally, condemned the move, saying it was a sign that the situation in the Balkans was deteriorating.
The vote to convert Kosovo’s lightly armed emergency response force into a professional army passed on December 14 with a unanimous vote by 105 deputies in the 120-seat assembly.
The session was boycotted by minority Serb lawmakers.
"Kosovo's parliament has adopted the law on the Kosovo Security Force! Congratulations!" parliamentary speaker Kadri Veseli said.
The legislation will double the size of the Kosovo Security Force and gradually transform it into a professional army of 5,000 troops.
The move has received support from all parties in the Western Balkan nation with an ethnic-Albanian majority except for lawmakers who represent the country’s 120,000-strong ethnic-Serb minority. Those lawmakers have boycotted parliament sessions on the matter.
President Hashim Thaci said that the new army will be "multiethnic, professional, and serve all citizens, peace in Kosovo, the region and wherever in the world when asked."
The United States has backed Kosovo's move, but it attempted to reassure those opposed to the action by insisting the process would take "many years.”
The U.S. ambassador to Pristina, Philip Kosnett, said that "it is only natural for Kosovo as a sovereign, independent country to have a self-defense capability."
"The United States reaffirms its support for the gradual transition of the Kosovo Security Force to a force with a territorial defense mandate, as is Kosovo’s sovereign right. The vote in the assembly today is the first step in developing this capability," the U.S. Embassy in Pristina said in a statement issued shortly after the vote.
The statement urged Kosovo to continue "close coordination with NATO allies and partners and to engage in outreach to minority communities."
The statement also said "regional stability requires that Kosovo make genuine efforts to normalize relations with its neighbor Serbia, and we encourage both sides to take immediate steps to lower tensions and create conditions for rapid progress on dialogue."
Serbia's prime minister said the formation of an army in Kosovo goes against efforts at stability in the Balkans.
Ana Brnabic said on December 14 that "Serbia will try to continue on the path of peace and stability, the road of prosperity."
"Today is not the day that contributes to cooperation and stability in the region," she added.
Officials estimate it will take up to a decade for the current Kosovo Security Force to become a combat-ready army.
NATO Secretary-General Jens Stoltenberg had stressed before the vote that Kosovo's plan was "ill-timed" and goes "against the advice of many NATO allies."
Reacting to the December 14 vote, Stoltenberg said NATO will "reexamine" its role in Kosovo.
"NATO supports the development of the Kosovo Security Force under its current mandate. With the change of mandate, the North Atlantic Council will now have to reexamine the level of NATO's engagement with the Kosovo Security Force," Stoltenberg said in a statement.
"I regret that this decision was made despite the concerns expressed by NATO," Stoltenberg added.
Meanwhile, the European Union's foreign policy chief has expressed regret over Kosovo's decision to form an army.
A December 14 statement from Federica Mogherini's office said the EU was in agreement with NATO that the mandate of Kosovo's current security force "should only be changed through an inclusive and gradual process in accordance with the Kosovo Constitution."
UN Secretary-General Antonio Guterres also expressed concerns about the move, spokesman Farhan Haq said.
"The secretary-general calls on all parties concerned to exercise restraint and refrain from actions that could raise tensions and cause a further setback in the European Union-facilitated dialogue for the normalization of relations between Belgrade and Pristina," Haq said
In a strongly worded statement, Russia's Foreign Ministry said, "It is evident that Kosovo is becoming an epicenter of instability, a source for conflict potential in the region."
The ministry accused the West of downplaying the shifts in the balance of power in the Balkans.
"The European Union has failed its mediatory role in dialogue between Belgrade and Pristina," it said.
Belgrade and ethnic Serbs in Kosovo have vehemently opposed the creation of a Kosovar military, saying it would violate UN resolutions and be used against the country's Serb minority -- a claim denied by officials in Pristina.
Nationalist Serbian newspapers have warned the move could set off a new conflict. The daily Informer stated that "War with Kosovo will start on December 15," the day after parliament’s vote.
Serbian officials have downplayed the possibility of war, but Serbian President Aleksandar Vucic said on December 13 that the "situation will be considerably worsened" if Kosovo goes ahead with the decision.
"We are not going to beat the war drums, but we will not allow anyone to purge and humiliate the Kosovo Serbs," Vucic said.
He also denounced the United States for its support of a Kosovo army and praised allies Russia and China for opposing the move.
Vucic addressed the nation, saying Kosovo and its "sponsor" the United States want to "quash" the Serbs, but that he won't allow it.
Serbia has been "brought to the edge" by Kosovo's decision, Vucic said, and now has no choice but to "defend" itself.
Russia and China have supported Serbia in its rejection of Kosovo's 2008 declaration of independence from Serbia. The United States and most of the West have recognized Kosovo's independence.
Relations between Pristina and Belgrade have been tense since Kosovo declared independence from Serbia in 2008. Although more than 110 countries recognize Kosovo, Serbia does not.
Serbia lost control over Kosovo in 1999 after NATO launched air strikes to stop the killing and expulsion of ethnic Albanians by Serbian forces during a two-year counterinsurgency war.
Nearly two decades after the end of the conflict, the landlocked territory of 1.8 million people is still guarded by NATO troops.
The current Kosovar security force has 2,500 members trained by NATO and tasked with crisis response, civil protection, and ordinance disposal.
Ahead of the vote, it held exercises in the south while NATO-led peacekeepers deployed a convoy of combat vehicles in the north of Kosovo.
Many of Kosovo's Serbs called it a provocation, but the NATO mission said it was a routine exercise.
With reporting by RFE/RL's Balkan Service, AFP, Reuters, and AP