French police have arrested the former prime minister of Kosovo based on a Serbian arrest warrant for alleged war crimes.
Ramush Haradinaj, 48, served as a rebel commander in the 1998-1999 Kosovo conflict, which eventually led to Kosovo declaring independence.
He twice faced war crimes charges at the UN, but was acquitted both times.
Kosovo's foreign ministry said his arrest was unacceptable, and that it was working to secure his release.
But Serbia said it would seek his extradition.
Mr Haradinaj, now an opposition leader in Kosovo, was arrested by police at Basel Mulhouse Freiburg airport, close to the Swiss and German borders in eastern France. He was due to appear at a court in Colmar on Thursday.
Authorities said they were acting on an arrest warrant from Serbia, apparently dating back to 2004.
French sources told Reuters news agency they would consider whether there were grounds not to act on the extradition request, particularly if it had been made for political reasons.
Serbian Prime Minister Alexander Vucic said on Thursday that there were numerous incriminating pieces of evidence against Mr Haradinaj and he hoped France would put justice ahead of politics.
The former rebel leader was briefly detained by Slovenian police in 2015 but soon released.
He was a commander in the Kosovo Liberation Army during the 1990s conflict, and Serbia alleges he oversaw a campaign of torture and murder against ethnic Serbs.
He has consistently denied the allegations, and stepped down as prime minister after just 100 days to face the charges.
The judge at his most recent trial in 2012 said there was no evidence to support the claims.
Kosovo declared independence from Serbia in 2008, but its status is not recognised by Serbia.