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Two Vietnamese men have been beheaded by Philippine Islamist group Abu Sayyaf, the country's military said.

Their bodies were found on Basilan island in the country's south, a stronghold of the group.

The two victims were part of six Vietnamese sailors kidnapped by the militants last year.

The southern Philippines have seen a surge in Islamist militancy with recent fighting in the city of Marawi claiming more than 400 lives since May.

"Our troops found the headless bodies after local residents alerted us," the Philippine military told the AFP news agency.

Three of the hostages are thought to still be held while one was freed last month.

Abu Sayyaf is one of the smallest and most violent jihadist groups in the southern Philippines, known for its kidnap-for-ransom activity and its brutality, including beheadings.

It has pledged allegiance to so-called Islamic State and has carried out kidnappings of both foreigners and Filipinos.

In February, the group beheaded a German hostage after their for demand ransom had not been met. In 2016, a Canadian hostage was beheaded.

According to the police, Abu Sayyaf is thought to be holding a total of 22 hostages, including 16 foreigners.

Abu Sayyaf was set up in the 1990s and is one of several groups that in recent years have declared allegiance to so-called Islamic State.

The southern Philippines is home to the largely Catholic country's main Muslim enclave and has a long history of separatist violence.

In the city of Marawi, fighting between a different Islamist group, the Maute group, and the military has been going on since May with the army still struggling to defeat the militants.