Philippine navy personnel stand onboard the yatch Rockall, where the body of a naked white woman who had been shot was found aboard, after it was in Sulu province

Islamic militants who specialise in kidnapping for ransom have reported killing a German sailor and abducting her elderly male companion from their yacht off the southern Philippines, the military said Monday.

A commander from the Abu Sayyaf group, which has earned millions of dollars by kidnapping foreigners and killed others after ransom demands were not met, claimed responsibility for the latest incidents, regional military spokesman Filemon Tan told AFP.

It appeared the victims were the same as those abducted by Somali pirates off the Gulf of Aden and held hostage for 52 days in 2008. The husband and wife were released at that time after a ransom was paid.

Tan said the military had recovered the couple's yacht, named the Rockall, and the body of a naked white woman who had been shot was aboard.

He said the military had listened to an audio recording of a known Abu Sayyaf leader claiming responsibility for the raid on the yacht, and the abducted German man also spoke on the phone call.

The abducted man identified himself as Jurgen Kantner, aged 70, and his companion as Sabine, according to Tan.

Tan said the passports of Kantner and Sabine Merz were found on the Rockall.

He said the photo in Merz's passport appeared to match the body of the dead woman aboard, although her identity still needed to be confirmed.

In Berlin the German government said it had yet to verify the media reports.

"We are trying right now together with Filipino authorities to determine (what happened)," foreign ministry spokesman Martin Schafer told reporters.

"We have to see what the facts are, to see if what the media are saying is actually true."

AFP interviewed Kantner in 2009 about being kidnapped along with Merz, identified then as his wife, by Somali pirates. He was subjected to a mock execution during the ordeal.

Speaking from Somalia where he had returned to retrieve their boat, Kantner said the threat of another kidnapping would not stop him from sailing.

"My boat is my life and I don't want to lose her, nothing more. I don't care about pirates and governments," Kantner told AFP from Berbera.

Kantner said then that he could not afford another kidnapping, after the ransom was paid to the Somali pirates.

"I really hope the pirates don't catch me because this time no one will pay and everybody will tell the pirates: 'Keep him'," he said.

- Growing threat -

The Abu Sayyaf, a loose network of militants based on remote islands in the southern Philippines, has defied more than a decade of US-backed military offensives and has been on a lucrative kidnapping spree in recent years.

In 2014 the militants abducted another German couple off a yacht in the southern Philippines. They released the pair six months later after receiving what they said was the full ransom demand of 250 million pesos ($5.1 million).

The Abu Sayyaf beheaded two Canadian men this year after demands for millions of dollars were not met.

The Abu Sayyaf is also blamed for kidnapping dozens of Indonesian and Malaysian sailors off the southern Philippines near where the Rockall was raided.

Most of those sailors were released after ransoms were reportedly paid.

However some are still being held and another two Indonesian fishermen were abducted off Malaysia's eastern state of Sabah at the weekend.

The Philippine military launched an offensive against the Abu Sayyaf after President Rodrigo Duterte vowed in August to eliminate the group.

But the Abu Sayyaf has continued to carry out raids apparently without hindrance, with the poorly equipped troops unable to secure the remote and mountainous southern islands or surrounding waters.

Foreigners typically stay away from that area because of the kidnapping threat.

However Islamic militants are also planning to kidnap foreigners in tourist hotspots of the central Philippines, the US government warned last week.