The Solar I oil tanker went down in rough waters off the island of Guimaras on August 11. Two crew men are still missing.
A local coastguard official said they did not know if they could contain the spill from the sunken vessel because they have not yet determined if the tanker is still leaking oil.
Harold Jarder, a coast guard commander, said: "Because we're not able to control the spill we have to continuously spray during daytime.
"That is the only thing that has helped a lot in keeping the oil from reaching the shoreline.
"The threat is still there. We are not even sure how much of the oil has been spilled."
Harder said it was a "necessity" that an underwater survey be conducted of the Solar I, which has been spotted lying 640m underwater around 13.3 nautical miles south of Guimaras, beyond the reach of human divers.
The oil spill has already polluted 300km of coastline, including large stretches of pristine beaches.
About 50,000 gallons of oil have spilled out from the tanker and environmentalists fear that the remaining 450,000 gallons still in the hold might also leak out.
Oil refiner Petron, the company that contracted the tanker, has hired a Japanese salvage ship equipped with a remote-control mini-submarine to help in the process, but its arrival has been delayed until Tuesday.
Two coastguard vessels, a tugboat and a BN Islander aircraft, contracted by the tanker's owner, have been spraying dispersant in the waters off Guimaras where the main oil slick could be seen.
A Guimaras resident removes his
gloves after helping the effort
Smaller slicks have also broken off and are floating out to other islands in the central Philippines.
Harder warned that the dispersant was very expensive.
"We're running out of funds to purchase the chemicals," he said.
The oil spill has wrecked Guimaras' tourism industry and threatened the livelihoods of hundreds of fishermen who have attempted to clean up the oil with hand tools and improvised oil booms made of bamboo and wild grass.