Seven people are missing after Argentine and Paraguayan flagged vessels collided in the Parana River Delta, Argentine coast guard officials said.
Security forces have detained three Paraguayans from the tugboat of that nationality, the Ava Payagua, that was carrying 12 people.
"The captain, the helmsman and another person in charge of manoeuvres have been detained," coast guard Major Sergio Gaetan said.
The collision between the Ava Payagua and the Argentine-flagged Rio Turbio occurred on Saturday morning.
The coast guard was working the area with divers that rescued one member of the Rio Turbio crew, who is injured, but the other seven, all Argentines, have not yet been found.
"It looks like the Paraguayan tug went out of control and crashed into the Argentine vessel, a sand barge carrying a full cargo that sank rapidly," Juan Carlos Pucci, secretary general of the river workers union, said in a statement.
Rescue work is being supervised by Argentine Security Minister Nilda Garre, the coast guard said in a statement.
The Paraguayan-flagged ship remains "at kilometer 103.3 of the Parana River on the Palmas arm of the Parana Delta, while the sand barge is apparently resting on the river bottom but is not spilling fuel," it said.
The federal court at the city of Campana is investigating the accident.