Unknown assailants fired shots at two police trucks that were traveling near Freeport McMoRan's gold and copper mine in Timika, Papua, on Saturday. No injuries were reported in the incident, according to Metro TV.

The Mobile Brigade (Brimob) trucks were en route to a gas station at Mile 39 near the Grasberg mine when the shots were fired, Metro TV reported. The officers returned fire, but the attacker or attackers were believed to have fled the scene immediately.

A following sweep of the area conducted by a larger force of Brimob officers turned up no suspects or evidence.

Saturday's attack took place three days after three Brimob officers narrowly escaped death after the car they were in was fired upon at Mile 35.

The National Police have declared that they are on the highest alert in Papua after a spate of increased violence in the restive region.

"Everyone is on alert, especially in Puncak Jaya district," National Police Chief Gen. Timur Pradopo said on Tuesday, a day after two men shot dead the head of the Mulia subdistrict police at the local airport.