Rescue teams are facing a “very hard” operation to find those still missing following the sinking of a superyacht off the coast of Sicily on Monday, a spokesperson for the onsite fire brigade teams has told ABC News.

Luca Cari said that the rescue operation for the six people still missing from the U.K.-flagged Bayesian yacht is ongoing as of Wednesday morning. The vessel was lost early on Monday in stormy weather around half a mile from the fishing village of Porticello, close to the city of Palermo. Fifteen people have so far been rescued and 1 body has been recovered.

“For us, it remains a rescue operation”, Cari told ABC News when asked if emergency services were transitioning to a recovery operation. Asked if there was any hope that the missing may be surviving thanks to air pockets inside the sunk vessel, Cari responded: “One can never exclude anything but it seems rather improbable”.

He added that 12 of the 18 divers leading rescue efforts on Wednesday are specialized cave divers who have extensive experience working inside caves.