Abu Sayyaf militants have decapitated seven hostages and sent their heads to troops on the southern Philippine island of Jolo, the military says.
Philippine troops have been battling militants in Jolo for many months
The men - six construction workers and a factory worker, all of whom were Christian - were seized on Monday near the town of Parang in Jolo.
The military has vowed to intensify its efforts to track down the group.
Philippine troops have been fighting Islamic militants holed up in Jolo's mountainous terrain for several months.
Abu Sayyaf is the smallest of four Muslim rebel groups in the Philippines, with about 400 members.
The group is thought to have links with both al-Qaeda and the regional militant group Jemaah Islamiah, and has been blamed for a number of kidnappings and bombings in the region.
Militants ordered civilians to deliver two of the heads to a military camp and the other five heads were left at a second camp later on Thursday, officials said.
The men's remains were recovered from a village on Friday.
Philippine President Gloria Macapagal Arroyo said that the killings demonstrated the group's ruthlessness.
"Abu Sayyaf's acts of terror will not go unpunished," she said.
Maj-Gen Ruben Rafael, head of military forces in Jolo, said the killings had been carried out in retaliation for the death of a militant commander.
The road workers' employer had refused to pay a ransom for the men, he added.
In recent months Philippine troops, aided by US counter-terrorism trainers, have carried out extensive work on the island to track down the militants.
Two of the group's senior leaders, Abu Sulaiman and Khaddafy Janjalani, have been killed but dozens of gunmen are thought to remain on the island.
Bali bombing suspects Dulmatin and Umar Patek are also thought to be in the region.
The island of Jolo is some 950km (600 miles) south of the Philippine capital, Manila.