Sarajevo  - The European Union Force (EUFOR) in Bosnia- Herzegovina continued an investigation Friday into the crash of a Spanish EUFOR helicopter that killed four in the hills of the country's north-west Thursday, with few details emerging as yet.

The BO-105 chopper with two Spanish pilots and two German officers went down near Banja Luka around noon on Thursday during a transport flight from Sarajevo, the Spanish Defence Ministry said in Madrid.

Bosnian media reported that the cause of the crash was probably a problem with the aircraft's engine.

As Sarajevo daily Dnevni avaz reported, a local woods warden Enver Kadric and a group of wood workers were the first to approach the crash site at the Jasen hill between the central Bosnian town of Travnik and the north-western city of Banja Luka.

"We heard a helicopter, but the sound of its engine was not usual. It was more like someone was hammering a piece of metal," Kadric told the daily.

"Then we heard the explosion and saw smoke. We went there and found the helicopter's wrack and four burnt bodies," he said.

Besides being difficult to access, the area where the helicopter crashed, according to the report, was also believed to be heavily mined, as the separation line between Bosnian Serbs and Muslims was there during the 1992-95 war in Bosnia-Herzegovina.

The report said helicopter crashed few metres away from trenches left after the war.

"The site of the crash must be full of mines and might be one of the most dangerous mine-fields in the country considering the coordinates," Vlado Micic of the regional Mine Action Centre (MAC) office said.

Spain commands EUFOR and has the largest contingent with some 350 of the nearly 2,200 troops serving in the 31-nation force, which the EU launched in 2004 to take over from NATO peacekeepers. (dpa)