June 12, 1999, the day when NATO entered Kosovo after a three-month assault on Serb forces, marks the moment Belgrade effectively lost control of its former province.

Clinton is beloved in Kosovo for championing that intervention, which forced Serbian troops to withdraw from their battle with ethnic Albanian separatists, a conflict which claimed 13,000 lives, mostly Kosovo Albanians.

On the 20-year anniversary, Kosovo leaders have been toasting to the liberation, with tributes on social media this week signed with the hashtag #Kosovo20YearsFree.

Yet Kosovo's path to statehood has been anything but smooth.

Belgrade still refuses to officially accept the independence its former province declared in 2008, undercutting Kosovo's efforts to gain global recognition.

While Kosovo has the backing of the US and most of the West, Russia and China also reject its statehood, effectively shutting it out of the UN.

Clinton, who is almost considered a founding father in Kosovo, will address crowds on Wednesday in the centre of Pristina, where there is a statue and a boulevard in his name.

After the American touched down in Pristina on Tuesday, Kosovo President Hashim Thaci honoured him with a medal, saying it was a "token of appreciation" for the "liberty he brought to us".

A smiling Clinton said he would "always be proud" of his contribution.

- Statue for Albright -

Wednesday's ceremonies will also see the unveiling of a new bust of Madeleine Albright, who was US Secretary of State at the time is set to attend the event.

But the mood will be gloomier in Belgrade, which still fumes over NATO's attacks.

On Monday, Serbian President Aleksandar Vucic slammed the 1999 intervention as "the 19 most powerful countries attacking a small country committed to freedom."

"They have caused us enormous damage from which we are still recovering," he added.

The NATO strikes battered Serbian military targets and infrastructure for 78 days, also killing an estimated 500 civilians, including Serbians, Albanians and Roma, according to Human Rights Watch.

That violence remains etched into the memory of many Serbians and damage to buildings can still be seen in parts of Belgrade.

The anniversary comes at a time when already dismal relations between Kosovo and Serbia have tumbled to a low point in recent months.

A series of diplomatic clashes have soured long-running talks aimed at reaching an accord.

On Monday, the current occupants of the White House seized on the occasion to urge Kosovo and Serbia to make progress on frozen EU-led talks aimed at normalising their ties.

"NATO's intervention brought an end to the (former Serbian president Slobodan) Milosevic regime's campaign of ethnic cleansing, which left thousands dead and over one million civilians displaced," the White House said.

"To pay respects to those victims, Kosovo and Serbia must recommit to a peaceful and prosperous future for all of their citizens. This requires putting aside extremist rhetoric and historical revisionism."

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