A cargo ship MV Ifsia Mahin carrying 400 metric tons of coal sank in Poshur River near Mongla Port on Tuesday.
Sources said the cargo ship floated away after the strong current of the river tore up the rope that tied it to the buoy.
10-12 other ships had the same condition as that of MV Ifsia Mahin, and were drifting in the river amid the strong current.
However, nine staff members and one security guard were able to swim shore.
According to the harbor department, the main channel remains safe as the ship sank outside the Poshur channel.
"MV Ifsia Mahin" master Shah Alam said they transferred about 400 metric tons of coal from a foreign ship to the cargo ship at Haribaria in Mongla Port on 28 March.
Due to strong current in the river, the rope tied to the cargo ship split from the buoy on Tuesday afternoon.
At that time 10-12 other ships floated away from the buoy while their shipmasters were trying to anchor at a safe place to avoid accident.
But Ifsia Mahin collided with one of the moving cargo ships, and ultimately sank in Katakhali area.
Earlier on 26 February, a cargo ship named MV BB-1147 went under water at Poshur channel near Mongla port with 848 tons of coal.