The International Court of Justice is due to rule on the legality of Kosovo's unilateral secession from Serbia.
Judges will issue a non-binding ruling on Serbia's claim that Kosovo's declaration of independence was a violation of its territorial integrity.
If the ICJ sides with Serbia, Kosovo could be pushed into negotiating a settlement. The opposite could see more countries recognising its independence.
Earlier, the US reaffirmed its "full support" for an independent Kosovo.
The commander of the Nato-led peacekeeping force meanwhile said its 10,000 troops were ready for any violence sparked by the ruling.
"On the field we don't have indications about nervousness, about any upcoming threat," said German General Markus Bentler of the Kosovo Protection force, K-for.
Serbian forces were driven out of Kosovo in 1999 after a Nato bombing campaign aimed at halting the violent repression of the province's ethnic Albanians, who constituted 90% of its two million population.
Kosovo was then administered by the UN until February 2008, when its parliament voted to declare independence.
Most countries do not recognise Kosovo as independent, but it is thought others might do so if the ICJ decides in its favour.
So far 69 of the UN's 192 countries are in favour of recognition. They include the US, UK, neighbouring Albania and Croatia.
Those opposed include Russia, China and Bosnia.
The EU is not united on the issue - Spain and Greece are among five of its 27 members to be opposed.
Serbia still regards Kosovo as part of its territory and challenged its legality at the ICJ at The Hague.
At the start of the deliberations last December, Serbia's representatives argued that the move both challenged its sovereignty and undermined international law.
Kosovo's representatives meanwhile warned that any attempt to reverse its independence might spark further conflict.
'Moment of truth'
Although non-binding, the court's ruling will provide a framework for diplomats to try to establish a working relationship between Serbia and Kosovo.
The dispute remains an obstacle to Serbia's hopes of joining the EU, and has hindered Kosovo's ability to attract foreign investment. Parts of northern Kosovo also remain tensely divided between ethnic Albanians and Serbs, and clashes occasionally erupt.
The ruling will also be closely watched by several other countries facing secessionist challenges themselves, such as China and Spain.
"The first and foremost consideration for any democratic government in the world is the preservation of its own sovereignty and territorial integrity," Serbian Foreign Minister Vuk Jeremic told the BBC.
"We do expect that the court is not going to endorse the legality of the unilateral act of secession, because if they do so then no border anywhere in the world where a secessionist ambition is harboured will ever be safe."
During a meeting with Kosovo Prime Minister Hashim Thaci in Washington on Wednesday, US Vice-President Joe Biden "reaffirmed the United States' full support for an independent, democratic, whole, and multi-ethnic Kosovo whose future lies firmly within European and Euro-Atlantic institutions", the White House said in a statement.
Mr Biden also "reiterated the United States' firm support for Kosovo's sovereignty and territorial integrity," it added.
A senior US official told reporters that it was confident the ICJ would rule in favour of Kosovo's declaration of independence.
The BBC's Mark Lowen in Belgrade says Serbia has called this "the moment of truth", and whatever the outcome it will be historic - the first time the ICJ has ruled on a case of territorial secession.