Photo posted of Germans believed kidnapped by Abu Sayyaf

ZAMBOANGA CITY, Philippines - A photograph of two German tourists believed kidnapped by an al-Qaeda-linked militant group has been posted on Facebook -- raising suspicion they are being held by the Abu Sayyaf in the Philippines' Muslim south.

In the undated picture, masked gunmen with heavy automatic weapons surround the sailors -- Stefan Viktor Okonek, 71, and Herike Diesen, 55 -- behind a German flag in a forest.

An Arabic inscription can also be seen in the background. It remains unknown who posted the photograph.

Okonek and Diesen were reportedly abducted by heavily armed men while headed to Malaysia’s Sabah from the nearby Philippine island province of Palawan on April 14.

Authorities said the two Germans had just come from an island-hopping trip around the coast of Mindanao -- the second largest and southernmost major island in the Philippines -- when they were snatched in the waters off Bataraza town.

Rescue operations are currently active in the island provinces of Sulu, Basilan and Tawi-Tawi -- areas where the group is known to operate.

Lt. Gen. Rustico Guerrero, chief of the army’s Western Mindanao Command, said they were facing difficulties as hostages were being used as human shields, and called on locals to provide information.

The German Embassy could not be reached Wednesday for comment on the authenticity of the picture.

However, Filipino construction workers who were released late last month had reported seeing five foreigners held captive by the Abu Sayyaf in Sulu.

They told reporters that two Germans who appeared to be a couple had been among those remaining.

Other captives are believed to include a 45-year-old Chinese businesswoman and her 19-year-old daughter, who were snatched from a karaoke bar in Basilan province May 22, and two European birdwatchers taken from Tawi-Tawi in February 2012.

A ransom of over $680,000 (P30 million) has been demanded for the mother and daughter.

Since 1991, the Abu Sayyaf -- armed with mostly improvised explosive devices, mortars and automatic rifles -- has carried out bombings, kidnappings, assassinations and extortions in a self-determined fight for an independent Islamic province in the Philippines.

It is notorious for beheading victims after ransoms have failed to be paid for their release.

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