ZAMBOANGA, Philippines (4th UPDATE) - Philippine authorities said Thursday they had failed to find two European birdwatchers in the crucial 24 hours after their abduction and warned Islamic militants may be holding them.
Hundreds of Marines joined the search for Swiss Lorenzo Vinciguerra, 47, and Dutchman Ewold Horn, 52, who were seized by armed men on the remote Tawi Tawi archipelago in the lawless south of the country on Wednesday.
"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."
Cabangbang said one of the groups that may be involved in the abductions was the Al-Qaeda-linked Abu Sayyaf, which is blamed for a spate of other kidnappings of foreigners in the south.
He said the navy was trying to block routes leading to Jolo, an Abu Sayyaf stronghold near Tawi Tawi where an army air raid on Thursday morning left 15 militants dead, including one of Southeast Asia's most wanted men.
Cabangbang said the first 24 hours were crucial in deciding the fate of people kidnapped in the area because this was when they were typically taken into the abductors' rugged jungle lairs on remote islands.
"If the trail goes cold, the chances of recovering them swiftly will vanish little by little," he said.
At least 10 other foreigners have been kidnapped in the south since the middle of 2010, in what is largely a ransom business with Abu Sayyaf and other militants demanding huge amounts of money for their captives' release.
Five of those kidnapped -- an Australian, two Malaysian traders, an Indian married to a Filipina and a Japanese man -- remain in captivity.
The military said some of those hostages were believed to be on Jolo, but their fate after Thursday's bombing raid was unknown.
Over the past decade, dozens of foreigners and locals have been kidnapped. Some of them, including an American, were beheaded after ransoms were not paid.
Cabangbang said the Jolo raid had been planned for months targeting leaders of the Abu Sayyaf and Southeast Asian terror network Jemaah Islamiyah, and was not directly connected to the kidnapping of the Europeans.
Ivan Sarenas, a Filipino guide for the two wildlife enthusiasts, was also kidnapped, but said he managed to jump off a boat that was taking the abducted men away.
"There was a passing boat and I decided to go for it. I held the barrel of the long firearm of the man in front of me with one arm and jumped out," he told AFP by phone.
He said Vinciguerra and Horn had travelled to Tawi Tawi, ignoring foreign government advisories about the kidnapping threat there because they wanted to see the Sulu hornbill, a critically endangered bird species.
"They are both into hornbills and they told me they wanted to see the rarest ones before they grew old," Sarenas said.
A colleague of Sarenas's from the Wild Bird Club of the Philippines said the Europeans were believed to have been taken by locals who apparently planned to hand them over to a militant group.
In the local kidnapping-for-ransom business, it is common for small abduction gangs to "sell" their captives to more powerful organisations such as the Abu Sayyaf.
The Swiss foreign ministry confirmed the incident and said it was doing "everything in its power to ensure that the hostages are released safe and sound".
A rotating force of 600 US troops has 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, partly because of their ability to raise funds through kidnapping-for-ransom operations. The Abu Sayyaf is believed to have just a few hundred militants.