A Vietnamese sailor kidnapped by Islamist militants off the southern Philippines has been rescued after nine months in captivity, the military said on Monday, following the beheading of two fellow crewmen in July.

Philippine troops rescued the hostage on Sunday on Basilan island in the southern Mindanao region, a stronghold of the Abu Sayyaf kidnap-for-ransom group, following intensified operations against the militants, authorities said.

"The hostage was able to run from the militants in the course of military operations and our troops rescued him with the help of local connections," military spokeswoman Captain Jo-Ann Petinglay told AFP.

"We have been conducting continuous operations against (the militants) involving airstrikes. The group was under pressure."

Petinglay denied a ransom had been paid for the Vietnamese hostage's freedom.

He and five and other crewmen were abducted in November from a Vietnamese cargo vessel sailing less than 20 kilometres (12 miles) from Basilan.

Militants beheaded two of the sailors last month, prompting President Rodrigo Duterte to threaten to eat the militants alive in retaliation.

Duterte often uses extreme language, when talking about Islamic militants.

One Vietnamese crewman is still in the hands of the Abu Sayyaf following the rescue of a sailor in June and the death of another during a gunbattle in July, Petinglay said.

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, and joined militants battling security forces since May in Marawi, the largely Catholic nation's most important Islamic city.

The militants continue to occupy parts of the southern city despite a US-backed military offensive there that has claimed more than 700 lives and displaced nearly 400,000 people.

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.

Abu Sayyaf militants are holding a total of 18 hostages, including 14 foreigners, Petinglay said.

source:AFP