Julia Campbell, 40, was last seen on April 8 in the town of Banaue in Ifugao province, where she had planned to hike alone, said U.S. Embassy spokesman Matthew Lussenhop.

The area, about 260 kilometers (160 miles) north of Manila, is famed for its mountainside rice terraces and pine forests. The communist New People's Army also operates there.

Lussenhop said the Peace Corps started looking for Campbell after she missed appointments on April 11.

"The Filipino authorities have been working very hard, using helicopters and local guides for a very extensive search," Lussenhop said. Security officers from the embassy and Peace Corps also were in the area to help, he said.

Regional police commander Chief Superintendent Raul Gonzales said at least four teams from the provincial police office had been mobilized for the search, after the U.S. Embassy told them Campbell was missing.

He said the directive to conduct the search came from the national police headquarters in Manila.

Maj. Gen. Rodrigo Maclang said members of an army company in Banaue also joined the search Saturday, after receiving an order from the military's Northern Luzon Command.

"We were unaware of the incident. We learned only today that someone has been missing," he said.

Maclang, commander of the 5th Infantry Division based in Isabela, east of Ifugao, said informants have told the military that Campbell had also traveled to the mountain tourist town of Sagada in nearby Mountain Province.

She was seen arriving on April 8 by motorcycle taxi at a Banaue road junction leading to nearby Batad village, from where she had planned to hike to a spot to view the rice terraces, he said.

Campbell also contacted a local masseuse to meet her at the Village Inn in Batad, but did not show up for her 6:30 p.m. Sunday appointment, Maclang said.

He said she also had a bus reservation to go back to Manila on April 9 because she had to catch a flight the next day. He did not know where she was headed from the Philippine capital.