Islamist militants have beheaded two Vietnamese sailors held hostage for eight months in the southern Philippines, the military said on Wednesday.
Philippine troops found the remains of the two hostages early Wednesday morning on the island of Basilan, a stronghold of the notorious Abu Sayyaf kidnap-for-ransom group, military spokeswoman Captain Jo-Ann Petinglay said.
"This is a desperate measure of the Abu Sayyaf Group because they see they have no gains from their kidnap-for-ransom activity," Petinglay told AFP.
Vietnam's foreign ministry said it was seeking confirmation from Philippine authorities, and called for heavy punishment for the killers.
"Vietnam strongly condemns all acts of kidnapping and barbarous and inhumane murder, and we are of the opinion that these acts must be heavily punished," spokeswoman Le Thi Thu Hang told AFP.
Abu Sayyaf, originally a loose network of militants formed in the 1990s with seed money from Osama bin Laden's Al-Qaeda network, has splintered into factions, with some continuing to engage in banditry and kidnappings.
One faction has pledged allegiance to the Islamic State group, with members among those holding parts of Marawi, the largely Catholic nation's most important Islamic city.
Militants continue to occupy parts of Marawi despite a US-backed military offensive there that has claimed more than 460 lives and displaced nearly 400,000 people since it began in May.
The two Vietnamese were seized last November along with four other Vietnamese crew members of a vessel that was boarded by the militants off the southern region of Mindanao, the military said.
One of the six crewmen was rescued last month and three remain in captivity, Petinglay said.
Abu Sayyaf militants are holding a total of 22 hostages, including 16 foreigners, according to Petinglay.
The Abu Sayyaf is known to behead its hostages unless ransom payments are made.
German national Jurgen Kantner, 70, was beheaded in February after the kidnappers' demand for 30 million pesos ($600,000) was not met.
Last year, the group beheaded two Canadian hostages.