GUWAHATI, India, May 14 (Reuters) - Five people were killed in India's northeastern state of Assam in clashes between rival ethnic groups, police said on Monday.

The killing of a civilian in an anti-insurgency operation triggered the violence.

A curfew was imposed in the far eastern town of Digboi after the violence, hampering work at oil fields in the region which is home to a state-run refinery.

Clashes erupted on Sunday when ethnic Assamese people blocked roads to protest against the killing of a civilian more than a week ago by security forces. An official probe has been ordered.

The blockade restricted the movement of non-Assamese tribespeople who work in tea gardens in the area. Armed with spears and bows and arrows, they tried to break up the roadblocks.

In the ensuing clashes, five people were killed, including two who were burnt to death.

"Two youths on their way to organise the roadblock at night were caught by a group of tea labourers and burnt alive," a police officer told Reuters by telephone, adding four Assamese were also missing.

Thousands of people have died in a separatist insurgency in oil- and tea-rich Assam in the past three decades.

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