A militant Muslim group says it planted a bomb on board a Philippine ferry which caught fire at sea, leaving about 180 people missing.
Witnesses said they heard an explosion before the fire
A man identifying himself as Abu Sulaiman of the Abu Sayyaf group made the claim in a call to a Philippine radio station.
Military authorities say they are sceptical about the claim and will await the results of an investigation.
Meanwhile, rescuers have boarded the wreck of the vessel for the first time.
The smouldering 10,000-ton Superferry 14 had been too hot to enter before Sunday.
Teams armed with axes and spotlights entered the ship through broken windows and explored the submerged command bridge but have not yet found anybody.
"We are not saying there are a lot of bodies yet," said coastguard Rear Admiral Arthur Gosingan.
"The ferry is not totally burned. We hope that we could still find survivors."
Relatives of the missing said they feared people were still trapped in their cabins.
There are conflicting reports about the number of people unaccounted for.
Authorities say some of those listed as missing might have been picked up by fishing boats after jumping into the water to escape the flames.
One person died and more than 700 were rescued, according to the coastguard and the ferry operator.
Radio Mindanao Network's programme director Benji Alejandro said Abu Sulaiman told him Abu Sayyaf attacked the ferry in revenge Filipino Muslims being hurt in Philippine military operations.
The fire broke out on the same day that a court sentenced two Abu Sayyaf members to life imprisonment for kidnapping.
Witnesses said they heard an explosion before the fire started shortly after the ferry set sail en route from Manila to Bacolod.
Investigators have not ruled out sabotage, but are sceptical that it was a bomb attack.
"They just want to ride and hitch on the media mileage on this Superferry fire," said Philippine military spokesman Lt Col Daniel Lucero.
The Abu Sayyaf, which mainly engages in kidnap for ransom, is one of four Muslim rebel groups operating in the southern Philippines.
It has been labelled a terrorist organisation by both Manila and Washington, and is believed by the US to have links with Osama Bin Laden's al-Qaeda network.
The Philippine Government has deployed thousands of troops in the south in an effort to rescue the last of the hostages and eradicate the Abu Sayyaf.