MANILA -- Ten people remained missing after an inter-island ferry with 344 people onboard sank early Monday in waters off Basilan province in the southern Philippines, with 18 having been confirmed dead, authorities said Tuesday.

The Philippine Coast Guard (PCG) on Tuesday revised the total number of people aboard the sunken ferry to 344, down from the initial count of 359, after verifying that 15 passengers who were listed as having boarded the vessel never did so.

A thin oil sheen was observed at the site where the ferry sank, the PCG said.

"This was assessed to have minimal environmental impact and is expected to naturally dissipate due to weathering processes such as evaporation and wave action, requiring no immediate response," the Coast Guard District Southwestern Mindanao said on social media.

In response to the tragedy, the Department of Transportation has suspended the entire passenger fleet of Aleson Shipping, the operator of the sunken vessel, pending a full investigation into the incident.