Philippine authorities said Thursday they had failed to find two European birdwatchers in the crucial few hours after their abduction, and warned Islamic militants may be holding them.

Hundreds of Marines had joined the search for Swiss national Lorenzo Vinciguerra, 47, and Dutchman Ewold Horn, 52, who were seized by armed men on a tiny island in the lawless south of the country on Wednesday, they said.

"There is a massive search and rescue operation right now to find the kidnappers and their captives," regional military spokesman Lieutenant Colonel Randolph Cabangbang told AFP.

"Though as of the moment we have not pinpointed their exact location."

Ivan Sarenas, a Filipino guide for the two wildlife enthusiasts, was also kidnapped, but managed to jump off a boat that was taking the abducted men away from the remote island and swam to safety, police said.

Another local guide had also escaped earlier and reported the crime to local authorities.

Cabangbang said it remained unclear who abducted the men, but noted previous high profile abductions in the south were carried out by the Al Qaeda-linked Abu Sayyaf group.

"We cannot rule out the possibility that the Abu Sayyaf is involved," he said. "However, I must stress there are other armed groups, including pirates, who also operate in these waters."

A rotating force of 600 US troops have been stationed in the southern region of Mindanao for a decade, helping to train local soldiers how to combat the Abu Sayyaf and other Islamic militants.

But the militants have remained a threat, funding their activities partly by kidnappings of foreigners as well as locals and extorting money for their release.

Wednesday's abductions lifted the number of foreigners kidnapped in the southern Philippines since the beginning of last year to at least 12.

Five of them -- an Australian, two Malaysian traders, an Indian married to a Filipina and a Japanese man -- are still in captivity. Three abducted Filipinos are also still being held.

It is believed they are being held on remote islands around the Sulu Sea, not far from where the Europeans were abducted, that are known Abu Sayyaf strongholds.

The Australian, 53-year-old Warren Rodwell, was kidnapped from his home in a southern town in December and appeared in a video released to media last month in which he said his abductors were demanding $2 million for his freedom.