TIMIKA, Indonesia, Sept 15, 2008 (AFP) - An explosion near an airport belonging to US mining giant Freeport in Indonesia's eastern Papua province could be linked to an earlier mortar blast, police said Monday.

The Sunday night explosion was the second in three days on the property of the Freeport Indonesia copper and gold mine, which has long been a source of tension in the province.

There were no injuries in either case, police said.

"Up to now, we don't know the type of explosives used," he said in Jakarta.

The incident happened outside the mine itself and caused no disruption to operations, Freeport spokesman Mindo Pangaribuan said.

Police said Friday they had found two old mortar rounds, one already exploded, on Freeport's massive property. They said they found a hot plate suspected to have been used to heat up the rounds, which caused no damage.

Nataprawira said police discovered a hot plate at the scene of the power plant blast as well.

"Maybe it's from the same group" as the mortar incident, he said.

The poorly-armed separatist Free Papua Movement (OPM) has waged a low-level insurgency for decades in the isolated province.

Barnabas Madacan, an OPM spokesman, denied any involvement in the latest incident, saying "that's not the way we're going to do things."

He said the OPM's fight "is without weapons."

Two American teachers and an Indonesian colleague who worked at the mine were shot dead during an ambush near the facility in 2002.

US and Indonesian investigators found that Papuan separatist rebels were behind the attack, but local rights groups have long maintained the military had a hand in the killings.

Critics accuse Freeport of not giving enough to the people of Papua in return for running the mine. They allege the mine causes pollution and that the military's protection of the site leads to human rights abuses.

The firm has disputed the claims.

Freeport Indonesia is majority owned by US-based Freeport McMoRan.