Four people have been killed and at least 14 injured in an explosion in eastern Colombia, police have said.

The victims - all civilians - died when the car bomb exploded on a rural road in Arauca state, they said.

No-one has admitted carrying out the attack.

Two left-wing rebel groups are active in the area and have carried out similar attacks in the past.

United States special forces are expected to arrive in Arauca this month to train Colombian troops to protect an oil pipeline that is the target of frequent rebel attacks.

At least 30 people were injured in a series of bomb blasts in the capital, Bogota, last month.

Uribe's pledge

The bomb exploded on a rural road near the town of Fortul in Arauca state, about 350 kilometres (220 miles) north-east of Bogota, police said.

All nine victims appeared to be from Fortul, said Arauca Governor Emiro Palencia.

Uribe: uncompromising pledge to end the civil war

Mr Palencia said he had called an emergency meeting of security officials to investigate the incident.

Arauca is one of three provinces which President Alvaro Uribe has vowed to rid of rebels, in an attempt to end almost four decades of conflict.

It has been designated a "Zone of Rehabilitation and Consolidation" - giving the security forces unprecedented powers to arrest without warrant, impose curfews, search property and restrict movement.

Thousands of people have been killed in violence linked to Marxist rebels, right-wing paramilitaries and government forces in Colombia's 38-year-old civil war.