"The vessel surfaced and it reported a breakdown. It was therefore asked to change course and go to Mar del Plata," said Gabriel Galeazzi, the head of the naval base in the northeastern city, located 400 kilometers (250 miles) south of Buenos Aires.
Enrique Balbi, a spokesman for the navy, meanwhile told a press conference in the capital that seven signals received by naval bases over the weekend were not attempted distress calls from the submarine, as previously hoped.
The ARA San Juan, a German-built diesel-electric sub, made its last contact on Wednesday.
A multinational air and sea search is under way with help from countries including Brazil, Britain, Chile, the United States and Uruguay.