Thai navy ships and helicopters searched on Monday for more than 30 sailors still missing more than 17 hours after their warship sank in rough seas in the Gulf of Thailand.

As of Monday afternoon, 75 sailors from the HTMS Sukhothai corvette had been rescued and 31 were still missing, the navy said. The high waves that caused the accident had lessened since Sunday night’s sinking, but were still high enough to endanger small boats, the navy said.