A sonar scanner was being used in the search on Thursday for a barge tug that overturned and sank in the deep and muddy Chao Phraya River on Wednesday, trapping the husband-and-wife crew inside the vessel.

The boat went down in fast-flowing water about 20 metres deep while manoeuvring with five laden barges near a pier in Nakhon Luang district of Ayutthaya province.

Representatives from the Marine Department, the navy and the Ayutthaya marine office drew up a search plan that used side scanning sonar to locate the sunken tug.

Phitsanu Seetapan, head of the Marine Department’s under water operations, said the place the tug went down was at least 20 metres deep. The very strong current could have swept the two missing crew away from the boat, or they might still be trapped inside, he said.

Scanning sonar could find objects on the bed of the river and ascertain the depth.

Divers were assisting in the search.

The tug driver, Somchai Tharnkul, 60,  and his wife Naruemol Tharnkul have not been seen since their barge tug sank in the Chao Phraya River opposite Wat Phanan Choeng Voraviharn in Ayutthaya province late on Wednesday morning.

The tug was pulling five cargo barges into a pier in Nakhon Luang district of the central province. It overturned as the towing cable pulled the stern underwater, and then sank, in the strong current at the confluence of the rain-swollen Chao Phraya and Pasak rivers about 11.15am.

A woman is heard saying in the clip "Careful, calm down" and shouting "Careful'' then screaming "The tug sinks'' before no voice is heard. (A clip posted on @Ayutthayaruamjai Facebook page)