It was not immediately clear what caused the Philippine-flagged vessel to sink but all 20 crew members were rescued before it went down.
Spilled oil had been detected on the shore and in coastal waters near more than 60 villages close to the site where the vessel is thought to have sank, the disaster agency said.
About 36,000 hectares of coral reef, mangroves and sea-grass were potentially in danger of being affected by the oil slick, according to marine scientists at the University of the Philippines.
Oriental Mindoro Governor Humerlito Dolor vowed to seek compensation for the damage and other expenses.
“Let me assure you, the damage done directly on the environment and on our people’s livelihood will be given corresponding compensation depending on what is stipulated in the compensation guidelines,” he told a briefing.
The governor was speaking at a briefing together with representatives of the tanker owner, RDC Reield Marine Services, and contractors hired for the cleanup operations.
The tanker’s owner has contracted local agencies, Harbor Star Shipping Services and Malayan Towage and Salvage, for the cleanup.
“The situation is very difficult … because of the weather. If sea conditions are bad, it is also unsafe for our contractors to work,” Rodrigo Bella, vice president of Harbor Star, told the media briefing.
The two contractors would shoulder all expenses initially, including paying residents hired for cleanup jobs, Dolor said.
The national government has also pledged to hire locals under a scheme to assist those whose livelihood has been affected by temporary fishing and swimming bans in affected areas.