The Split County Court confirmed that a Croatian man and a Serbian woman were placed in pre-trial detention on Wednesday as part of an investigation into espionage but provided little further detail.

According to local media, the Croatian army pilot and his girlfriend were arrested on suspicion of sending details on troop movements in Kosovo to a Serbian group in the country over the past three years.

"We take these allegations very seriously. The safety of KFOR personnel and the integrity of classified information are paramount," a NATO official told AFP.

The international troops have remained in Kosovo since the end of the 1998-1999 war between ethnic Albanian guerillas and Serbian forces.

Kosovo broke away from Serbia and declared independence in 2008.

Belgrade has not recognised this move, encouraging the Serbian minority both politically and financially to remain loyal to Serbia.

Most of the remaining Serbs in Kosovo live in several northern municipalities, where tensions often flare into violent incidents.

The NATO official said the troops would continue the "long-standing United Nations mandate to contribute to a safe and secure environment for all people and communities living in Kosovo".

Split county State Prosecutor's Office confirmed the investigation but said details would remain confidential.

According to Croatian state news agency HINA, the couple is being investigated by civil and military police, as well as Europol.

The man, who was reportedly deployed with KFOR, is suspected of sharing secret information about the movement of international troops in north Kosovo with his partner.

She is suspected of passing it on to the Serb List, a political party of Serbs in Kosovo backed by Belgrade.

"We are fully prepared to cooperate with Croatian authorities for the further development of investigations and the full clarification of their activities and operational network," Kosovo government spokesman Perparim Kryeziu said.

Earlier on Thursday, Croatian President Zoran Milanovic also backed the investigation.