The device, believed to have been placed by outlawed Kurdistan Workers' Party (PKK) rebels, exploded as the bus with a civilian licence plate was passing, the source said on condition of anonymity.

The soldiers, all dressed in civilian clothes, were on their way back to their barracks in Tunceli province after a medical check-up in the neighbouring province of Elazig.

One of them was seriously wounded in the attack, which also injured the nine-year-old son of the bus driver, the source added.

The Turkish army often uses civilian vehicles to transport troops in the east and southeast where the PKK has been fighting for self-rule for more than two and a half decades.

The rebels frequently employ remote-controlled mines and home-made bombs in attacks targeting Turkish security forces.

In a separate development, the Anatolia news agency reported Friday that police had arrested four suspects with suspected links to the PKK as they prepared for a bomb attack in the western city of Izmir.

Police found a bomb in the car in which the suspects were caught, the report said. The arrests took place in the city's Buca district, which has a large Kurdish community, added the agency, without saying when they happened.

The rebels have in the past carried out bombings in cities and tourist resorts in the country's west.

The arrival of spring has brought a rise in violence between Turkish forces and PKK rebels operating out of bases in Turkey and neighbouring northern Iraq.

Some 45,000 people have been killed since 1984 when the PKK, listed as a terrorist group by Turkey and much of the international community, took up arms for self rule in Turkey's Kurdish-majority southeast.