U.S. officials in the Philippines have confirmed that human remains found on a southern island last week are those of a U.S. citizen kidnapped in May by the Abu Sayyaf rebel group. The announcement comes as three Philippine hostages managed to escape from the rebels.

The spokeswoman for the U.S. Embassy in Manila Friday said the remains have been identified through dental records as those of Guillermo Sobero, a 40 year-old man from the western state of California. "The results of the forensic testing by the U.S. Army central identification laboratory in Honolulu, Hawaii, confirmed that the remains that have been recovered on Basilan in southern Philippines by the Philippines military authorities last week are those of Mr. Sobero", the spokeswoman said.

The Philippine military found the remains lying in a shallow grave after former members of the rebel group gave the location. Abu Sayyaf guerrillas, who say they are fighting to establish an Islamic state on the largely Muslim island of Basilan, announced last June that they had beheaded Mr. Sobero. They kidnapped him in May along with 17 Philippines and two other Americans. Since then, seven thousand Philippine soldiers have been hunting the guerrillas, who have conducted a series of high-profile kidnappings for ransom.

U.S. spokeswoman Karen Kelly condemned the murder. "The United States strongly condemns this act of terrorism by the Abu Sayyaf group against an innocent American citizen hostage. We hold the terrorists responsible for this despicable act of violence," she said.

The U.S. official confirmed that a team of about two dozen U.S. military advisers are to visit the southern Philippines soon to train and equip Philippine forces fighting the Abu Sayyaf. She said it was part of longstanding cooperation between the two governments in the fight against terrorism.

The announcement came as three of the hostages held by the Abu Sayyaf escaped Friday and were rescued by the military. A fourth escaped last Sunday.

The former hostages, identified as Philippine farmers, said they were tired and weak from marching continuously with their captors to avoid the Philippine army. They confirmed that the other hostages, including an American couple, were still alive.

The Philippine army has vowed to run the rebels to ground, but says its operation has been hindered by heavy rains in the dense jungle area.