ILIGAN CITY, Philippines -- (UPDATE) About half of the southern island of Mindanao lost power for several hours Friday morning after unidentified armed men blasted two power pylons of the National Transmission Corp. (Transco) in Lanao del Norte and Lanao del Sur, officials said.

The near simultaneous attacks in Bubong, Lanao del Sur and Mentering, Lanao del Norte brought to six the number of Transco towers attacked this week.

Elizabeth Ladaga, Transco communication officer, said the attack on the two pylons rendered eastern and southern Mindanao without power starting 9:27 a.m.

Ross Luga, information officer of the Davao Light and Power Company (DLPC) in Davao City, said power was gradually restored in some areas, including hospitals, as soon as the Transco supply was cut off.

Luga said DLPC, an Aboitiz company, used its generator sets to partially restore power to important facilities.

Ladaga said armed men have been bombing Transco towers for still unclear reasons since Tuesday.

On Thursday, Transco towers in Tingintingin, Kauswagan town and in Babalaya, Bacolod, both in Lanao del Norte, were also blasted.

In Alabel, Sarangani province, unidentified suspects bombed two Transco towers in Baluntay village shortly before midnight Tuesday.

Sarangani Governor Miguel Dominguez branded the attacks economic sabotage.

He said residents of the province would be experiencing power interruptions over the next four days while repairs are undertaken.

Belinda Canlas, Transco communication officer, placed the damage to Transco towers No. 40 and 39, which engineers are now working on, at around P5 million.

Senior Superintendent Danilo Peralta, Sarangani police director, told reporters police bomb experts recovered parts of a cellular phone, battery, and electrical wiring from the site of the explosions in Alabel.

But he said they were still trying to determine the type of explosives used in the attacks.

"We're still conducting investigation to determine whether this is an act of harassment, extortion or terrorism," Peralta said.

From January to February this year, Transco said 17 towers had been damaged by attacks in Mindanao.

Dawis said the company had to spend more than P10 million to restore the pylons.

In past attacks against Transco facilities, authorities cited extortion as the most likely motive.