By Tim Johnston BBC News, Jakarta

Three girls have been beheaded and another badly injured as they walked to a Christian school in Indonesia.

They were walking through a cocoa plantation near the city of Poso in central Sulawesi province when they were attacked.

This is an area that has a long history of religious violence between Muslims and Christians.

A government-brokered truce has only partially succeeded in reducing the number of incidents in recent years.

Police say the heads were found some distance from the bodies.

It is unclear what was behind the attack, but the girls attended a private Christian school and one of the heads was left outside a church leading to speculation that it might have had a religious motive.

Islamic state

Central Sulawesi and Poso in particular was the scene of bitter fighting between Muslims and Christians in 2001 and 2002.

Flashpoints: Sulawesi

More than 1,000 people were killed before a government-brokered truce.

Although the violence has been subdued, it has never gone away completely.

A bomb in May in the nearby town of Tentena, which is predominantly Christian, killed 22 people and injured over 30.

The fighting four years ago drew Islamic militants from all over Indonesia and many have never gone home.

Analysts say the militants have targeted central Sulawesi and believe that it could be turned into the foundation stone of an Islamic state.

The analysts have warned that the violence could resurface at any time.