Kosovo passed laws to build an army, asserting its statehood in a US-backed move that prompted outrage in Serbia, which does not recognise its former province's independence.
Serbia on Friday threatened a possible armed intervention in Kosovo after the Kosovo parliament on Friday overwhelmingly approved the formation of an army.
Belgrade called the move a "direct threat to peace and stability" in the Balkans and lashed out at the United States for supporting it.
While NATO's chief called the action by Kosovo "ill-timed," the US approved it as "Kosovo's sovereign right" as an independent nation that unilaterally broke away from Serbia in 2008.
Serbia insists the new army violates a UN resolution that ended Serbia's bloody crackdown on Kosovar separatists in 1998-1999.
It has warned bluntly that it may respond with an armed intervention in its former province, with Prime Minister Ana Brnabic saying that's "one of the options on the table."
On Friday, Nikola Selakovic, an adviser to the Serbian president, said the country could send in armed forces or declare Kosovo an occupied territory.
UNSC session sought
Serbian Foreign Minister Ivica Dacic said Serbia will seek an urgent session of the UN Security Council over the issue.
The Security Council held closed consultations late Friday on the format of a meeting, possibly on Monday or Tuesday.
Council diplomats, speaking on condition of anonymity because talks were private, said Russia, a close ally of Serbia, wants an open meeting to be addressed by Serbia's president while European nations want a closed session.
The decision will be made by Ivory Coast's UN ambassador, the current council president, the diplomats said.
UN Secretary-General Antonio Guterres affirmed the UN's desire to maintain the Kosovo Force as the body that ensures the safety of Kosovo, UN deputy spokesman Farhan Haq said late Friday.
He 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 normalisation of relations between Belgrade and Pristina."
Serbian President Aleksandar Vucic visited Serbian troops near the border with Kosovo and later Vucic addressed the nation, denouncing the United States for its apparent support of a Kosovo army and praising allies Russia and China for their opposition to the move.
He said that Kosovo and its "sponsor" - the US- want to "quash" the Serbs, but that he won't allow it.
Vucic says Serbia has been "brought to the edge" by Kosovo's decision and now has no choice but to "defend" itself. It was one of the strongest anti-American outbursts by Vucic, a former pro-Russian ultranationalist turned alleged pro-EU reformer.
Any Serbian armed intervention in Kosovo would mean a direct confrontation with thousands of NATO-led peacekeepers, including US soldiers, who have been stationed in Kosovo since 1999.
Russia denounced the move to form a Kosovo army, saying the ethnic Albanian force must be "disbanded" by NATO in Kosovo.
Kosovo declared independence from Serbia in 2008, a move not recognised by Belgrade or Russia.
Tensions have remained high between the two sides, and NATO and the European Union which has led years long talks to improve ties between the Balkan neighbours expressed regret that Kosovo decided to go ahead with the army formation.
"I reiterate my call on both Pristina and Belgrade to remain calm and refrain from any statements or actions which may lead to escalation," NATO Secretary-General Jens Stoltenberg said.
He said the alliance remains committed "to a safe and secure environment in Kosovo and to stability in the wider Western Balkans." He said they will "re-examine the level of NATO's engagement with the Kosovo Security Force."
The new army will preserve its current name Kosovo Security Force but now has a new mandate. In about a decade the army expects to have 5,000 troops and 3,000 reservists, and a $111 million annual budget.
On Friday, all 107 lawmakers present in the 120-seat Kosovo parliament voted in favour of passing three draft laws to expand an existing 4,000 Kosovo Security Force and turn it into a regular, lightly armed army. Ethnic Serb lawmakers boycotted the vote.
The new army will handle crisis response and civil protection operations essentially what the current paramilitary force, which is lightly armed, does. Its main tasks would be search and rescue, explosive ordnance disposal, firefighting and hazardous material disposal.
It's not immediately clear how much more equipment or weapons the new army will have or need compared with the current force.
Seeking to reassure Serbia and the international community, Kosovo's Prime Minister Ramush Haradinaj said the new army "will never be used against them (Serbs)."
He added: "Serbia's army will now have a partner -Kosovo's army- in the partnership for peace process."
Kosovo President Hashim Thaci said that the new army will be "multiethnic, professional and will serve all citizens, peace in Kosovo, the region and wherever in the world, when asked."
He called on a return to dialogue for normalising ties with Serbia.
US hails vote
The United States hailed Kosovo's parliament vote to form a new army as a first step and reaffirmed "its support for the gradual transition ... to a force with a territorial defence mandate, as is Kosovo's sovereign right."
A US embassy statement in Pristina urged Kosovo to continue "close coordination with NATO allies and partners and to engage in outreach to minority communities."
"Regional stability requires that Kosovo make genuine efforts to normalise relations with its neighbour Serbia, and we encourage both sides to take immediate steps to lower tensions and create conditions for rapid progress on the dialogue," it said.
In a sign of defiance, Serbs in northern Kosovo displayed Serbian flags on their streets and balconies. NATO-led peacekeepers deployed on a bridge in the ethnically divided northern town of Mitrovica to keep the peace.
Kosovo's 1998-1999 war ended with a 78-day NATO air campaign in June 1999 that stopped a bloody Serbian crackdown against ethnic Albanian separatists.