Hundreds of naval troops and Marines have been deployed to search for Swiss Lorenzo Vinciguerra, 47, and Dutchman Ewold Horn, 52, in the remote Tawi Tawi archipelago, said Colonel Jose Johriel Cenabre.
The pair were snatched by an unknown group of gunmen on Wednesday while on an expedition to photograph rare hornbills in the wild, but government forces have since found no sign of the pair.
"Our search efforts have intensified. There is no reason for us to believe that they have slipped past the naval blockade (around Tawi Tawi)," said Cenabre, deputy commander of the local Navy.
The authorities have not been able to pinpoint who carried out the abduction and where the captives are being held. There are vast seas around Tawi Tawi, which consists of more than 300 small islands bordering Malaysia.
"We do not know who they are what their demands are," provincial police chief Senior Superintendent Rodelio Jocson told AFP.
"The group has not contacted us and we are still searching in the area.
"They (the kidnappers) have not said anything and we have not identified the group," said Major General Noel Coballes, the regional military chief.
The southern Philippines has long been plagued by groups of outlaws who kidnap people to hold for huge ransoms.
The most feared of these is the Al-Qaeda-linked Abu Sayyaf which has been tied to the worst terror attacks in Philippine history.
In previous cases, armed gangs have turned their captives over to Abu Sayyaf, who have been known to behead their captives.
But Abu Sayyaf is not widely active in Tawi Tawi, raising hopes that the hostages may still be in the hands of ordinary criminals.
Abu Sayyaf, founded with seed money from Al-Qaeda chief Osama bin Laden in the 1990s, are based largely in the islands of Jolo and Basilan to the northeast of Tawi Tawi.
US troops have been based in the southern Philippines for a decade to help train local troops in hunting down members of the group.