A pregnant passenger is missing after a boat capsized on a river in Thailand.

Phennapaporn Maksakhon, 21, was on the towing barges with her husband Sirawut Phobangwai, 24, when their tugboat flipped over while turning a river bend on Tuesday.

The mother-to-be, who was seven months pregnant, was reportedly trapped in the cabin and swept away as the boat was swallowed by the Pa Sak River in Ayutthaya.

Footage shows the vessel spurting out jets of water as it was submerged. Sirawut later swam to the surface, but his wife was nowhere to be found.

Police Captain Rung Panya of the Nakhon Luang district station said rescuers were dispatched to the river after being notified.

The sunken tugboat was towed back to shore. More than 20 divers then scoured the water but failed to find the missing Phennapaporn despite nine hours of searching.

Frantic husband Sirawut said his wife had been accompanying him while he was dragging an ore barge to a pier.

He said: 'At the time of the accident, I was at the rear of a convoy with five other tugboats in front of me. I was taking the barge from Wat Phanan Choeng Worawihan Pier towards Jumbo Pier but the boat tilted when I lost control.

'I was able to jump out in time but my wife couldn't because she was heavily pregnant.'

Peerathon Naksuk, director of the Provincial Marine Office, said the incident occurred just days after the channel was reopened due to strong currents.

He added: 'The cause is likely due to the boats entering a curve in the river, causing them to pull against each other. Based on the video, it appears to be caused by a twisting rope. Initially, one person is missing and a search is underway. However, caution is advised during the high tide.'

Authorities said the search for Phennapaporn was ongoing.