MANILA, Philippines - Philippine troops have found the remains of a German hostage who was beheaded by Abu Sayyaf militants in the country's south, vowing Sunday to rescue more than 30 other captives and crush the ransom-seeking extremists.

Marines dug up the head and body of Jurgen Gustav Kantner late on Saturday in the mountainous hinterlands of Indanan town in Sulu province, where the militants are holding at least 31 others, said regional military commander Major General Carlito Galvez Jr.

The 70-year-old Kantner was seized from his yacht with his female German companion off Malaysia's Sabah state in November. Kantner's companion was fatally shot on the yacht, which was later found in the southern Philippines, according to the military.

The couple had survived a kidnapping ordeal off Somalia in 2008.

"Once again, the command is sending its deep regrets to the family for not being able to rescue Mr. Kantner on time," Galvez said. He repeated a pledge to rescue other hostages and crush the Abu Sayyaf.

President Rodrigo Duterte's spokesman, Ernesto Abella, said the government "will leave no stone unturned in squarely addressing the evils of extremism and plain banditry".

"Rest assured these mindless acts will not go unpunished," Abella said.

German Chancellor Angela Merkel has condemned Kantner's killing as an "abominable act".

The Abu Sayyaf circulated a video of the beheading online.

Duterte has said Filipino forces tried their best but apologized to Germany and Kantner's family after troops failed to rescue him in his nearly four months of jungle captivity in Sulu, a poor Muslim province 950 kilometers south of Manila.