The death toll from the sinking of a tourist boat off Thailand’s Phuket island has risen to 33 as rescuers deployed helicopters to search for 23 people still missing.

The Phoenix capsized and sank in rough seas when it was toppled by 5m high waves on Thursday with 105 passengers on board, including 93 Chinese tourists and 12 Thai crew and tourist guides.

The Royal Thai Navy said “quite a few bodies” had been found inside the sunken vessel, while others were found floating in the water.

Rear Admiral Chareonpol Kumrasee told Reuters on Friday that the chances of survival for the missing were low.

The bodies of nine Chinese tourists were being brought to Phuket, where authorities had already confirmed the death of another Chinese man.

The Chinese embassy in Bangkok said in a statement that it had requested the Thai government put every effort possible into the rescue and that it had sent a team to Phuket to help.

The police are investigating the incident, the Phuket Marine Police said, adding the boat was properly registered and was not overloaded at the time of the accident.

It went down about 7km from the shore and sank to a depth of about 38-40 metres, the Royal Thai Marine Corps said in a statement.

Monsoon season

Phuket officials had earlier issued a warning of severe weather until Tuesday, including heavy rain and winds.

Thailand is in the middle of its monsoon season, which begins in May and usually ends in October and often generates high winds and flash storms in coastal areas.

“All agencies are co-operating in their search at this time,” Phuket Governor Norraphat Plodthong said. “We have given warnings … but the winds are strong.”

He said officials would consider whether to ban boats from going to sea during strong winds.

Another boat overturned off Phuket on Thursday. Officials said all 42 on board have been rescued.

More than 20 people were killed when a tourist boat carrying 150 people capsized in Thailand’s ancient city of Ayutthaya in 2016. Officials blamed the accident on overcrowding and reckless piloting.

The Thai incidents came after an overloaded boat carrying illegal Indonesian immigrants capsized in bad weather off Malaysia’s southern Johor state late on Sunday.

The Malaysian coastguard said on Friday that 25 people, including a woman, had been rescued. Ten others, including four women, were found dead. Rescue operations involving 150 personnel are ongoing for nine people believed to be still missing.