The US 13th Marine Expeditionary Unit said a member, who was not named, may have gone overboard Thursday morning.

Aircraft on board the USS Essex are searching waters off the Sulu Sea and the Surigao Strait while multiple searches are also being made inside the ship itself, the unit said on its social media sites late Friday.

The ship was conducting "routine operations in the Sulu Sea" at the time, the California based unit said.

"We remain committed to searching for and finding our marine," the statement quoted the marine unit's commander, Colonel Chandler Nelms as saying.

"All of our sailors, marines and available assets aboard the USS Essex have been and will continue to be involved in this incredibly important search and rescue operation," added the search commander, US Navy Captain Gerald Olin.

The Philippine coastguard are also taking part in what is now an expanded search covering about 3,000 square nautical miles (10,290 square kilometres), it added.

Sulu Sea, the Philippines' largest inland waterway, sprawls over the central and southern sections of the Asian archipelago, a US military ally.

Surigao Strait is at least 300 kilometres (186 miles) away, off the country's eastern seaboard.

The statement did not explain why the two separate locations were being searched.

Philippine navy and coastguard spokesmen in Manila told AFP Saturday they did not have a statement to make on the search.