Image copyright AFP Image caption Jurgen Kantner and his wife Sabine went back to Somaliland in 2009 to collect their boat

A German yachtsman is believed to have been abducted by Filipino Islamist militants, the Philippines' army says.

The naked body of a woman found on an abandoned yacht is thought to be his wife, army spokesman Filemon Tan said.

In an audio message, the leader of the Abu Sayyaf group, which often kidnaps for ransom, said it was behind the raid, the military spokesman added.

The couple, Jurgen Kantner and Sabine Merz, were held captive for more than a month by Somali pirates in 2008.

Their passports were found on board the yacht, Mr Tan said.

The German authorities have yet to confirm the incident.

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

In recent months it has been behind a series of attacks at sea between the Philippines and Malaysia, with tug boats and fishing vessels intercepted and their crews kidnapped and held for ransom.

The German couple's yacht, the Rockall, was found off Laparan Island in southern Sulu Province, Mr Tan said.

The woman on board had been shot dead - it was not clear why she was killed but she might have fought back, military officials said.

According to Abu Sayyaf, the couple were cruising off Malaysia's Sabah state when the militants intercepted them, the Filipino military said.

Mr Kantner, 70, and his wife were held for 52 days in 2008 by Somali pirates - and were released after a ransom was paid, the AFP news agency reports.

"My boat is my life and I don't want to lose her... I don't care about pirates and governments," Mr Kantner told AFP in 2009 when the couple returned to the self-declared republic of Somaliland to collect their yacht.

No ransom appeal

Philippine President Rodrigo Duterte is due to hold talks with Malaysian Prime Minister Najib Razak this week - and tackling Abu Sayyaf is expected to be on the agenda, the Reuters news agency reports.

Image copyright Abu Sayyaf Image caption Abu Sayyaf has pledged allegiance to so-called Islamic State

Mr Duterte, who came to power in June, has vowed to eliminate the group - and the military has launched a major military offensive against the militants.

According to a confidential government report, seen by the Associated Press news agency, the insurgents have made at least $7.3m (£5.8m) from kidnappings for ransom in the first six months of this year.

Mr Tan appealed for people to adhere to the Philippine government's no-ransom policy.

"If we give in to ransom, a greater damage will be done. They can use the money to buy arms and to feed their bandits and that will fuel again the tendency for them to kidnap," AP quoted him as saying.