A battalion of the Kosovo Security Forces standing in one place at the flag roll call this week (Picture: PA).
Kosovo's parliament has overwhelmingly approved the formation of an army, angering Serbia which talked up the possibility of an armed intervention in response.
The 120-seat parliament voted with all present 107 legislators in favour of passing three draft laws to expand the existing 4,000 Kosovo Security Force and turn it into a regular lightly armed army.
Ethnic-Serb community members boycotted the vote.
In a statement, NATO's chief called Kosovo's move "ill-timed" and urged dialogue to maintain peace.
Jens Stoltenberg also tweeted that "all sides must ensure that today's decision will not further increase tensions in the region."
Serbia insists the new army violates a UN resolution that ended Kosovo's 1998-99 war of independence.
It has warned bluntly that it may respond to the move with an armed intervention in the former province, with prime minister Ana Brnabic saying it was "one of the options on the table".
Nikola Selakovic, an adviser to the Serbian president, said the county could send in Serbian armed forces or declare Kosovo an occupied territory.
Serbian foreign minister Ivica Dacic described the move as "the most direct threat to peace and stability in the region and to the security of the Serbian people", and said the country will seek an urgent session of the UN Security Council.
In Serb-dominated northern Kosovo, Serb leader Goran Rakic said the new army was "unacceptable" and "showed clearly that Pristina does not want peace". He urged Serbs in Kosovo to show "restraint and not respond to provocations".
Kosovo declared independence from Serbia in 2008, a move not recognised by Belgrade or its ally 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 had decided to go ahead with the army formation.
The new army will preserve its former name - Kosovo Security Force - but with a new mandate.
In about a decade, the army will have 5,000 troops and 3,000 reservists, essentially operating as a security force handling crisis response and civil protection operations.
Seeking to reassure Serbia and the international community, Kosovo's prime minister Ramush Haradinaj said the 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 the peace process and it won't be a long time before we serve together."
Serbia fears the move's main purpose is to chase the Serb minority out Kosovo's Serbian-dominated north, a claim strongly denied by Pristina.
Kosovo's 1998-99 war ended with a 78-day NATO air campaign in June 1999 that stopped a bloody Serbian crackdown against ethnic Albanian separatists.