Seven people were missing on Saturday after an Argentine cargo ship and a Paraguayan tug boat collided on the Parana River near the northeastern city of Zarate in Argentina.
"We will continue searching for the seven who went missing overnight because the weather conditions allow it," said Sergio Gaetan, who is heading the search and rescue operation.
The missing are all from the Argentine boat, which capsized and sank after the crash.
The collision occurred at 4am local time (9pm New Zealand time) at the 102-kilometre mark of the Parana River, which runs through Argentina, Brazil and Paraguay. After they were alerted of the crash, divers rescued a crew member from the Argentine vessel who was slightly injured.
Gatean said police have arrested three of the commanders of the Paraguayan ship, out of a total of 12 crew members, while the tug boat will be "subject to judicial inspection."
"The time factor and the conditions of the crash jeopardise chances that the missing will survive, but we are not losing hope," said police chief Carlos Rios.
Rios said 16 divers are searching for the missing, but that all of the ship's compartments were flooded. The search also involves nine coast guard ships and a helicopter.