Argentina's navy says its ARA San Juan submarine, which has been missing since Wednesday, reported a mechanical breakdown in its last communication.
The submarine, with 44 crew on board, disappeared 430km (267 miles) off the Argentine coast and no trace of it has been found.
"The vessel surfaced and it reported a breakdown," naval commander Gabriel Galeazzi said.
He did not give further details of the nature of the breakdown.
This is the first time that an official has mentioned the sub encountering mechanical problems.
However, the brother of a crew member had earlier told local media that in a message before communications were lost his sibling had mentioned that the vessel was having problems with its batteries.
Capt Galeazzi, who heads the naval base in Mar del Plata, south of Buenos Aires, said that mechanical problems were not uncommon and rarely posed a risk.
The naval commander said that the submarine had been asked to cut short its patrol and go directly to Mar del Plata.
According to local media, the captain of the ARA San Juan contacted the naval base again after reporting the mechanical problem.
In the message, the sub's captain reportedly said he was heading towards Mar del Plata with all 44 crew members in perfect health.
Signals not from sub
The navy also announced on Monday that seven signals picked up at the weekend were not from the missing submarine's satellite phone.
The failed calls, lasting between four and 36 seconds, had been received on Saturday. They had raised hopes that the San Juan's crew members were alive.
US satellite company Iridium had earlier said that the submarine carried one of its satellite phones on board.
But the company said that the seven signals did not come through its network and, on Monday, navy spokesman Enrique Balbi confirmed the attempted calls "did not come from the submarine's satellite phone".
A huge search and rescue operation is continuing in the South Atlantic.
Specialist underwater rescue equipment has arrived in Argentina from the United States and more boats and planes have also joined the search, which has been hampered by heavy winds and high waves.
The ARA San Juan was returning from a routine mission to Ushuaia, near the southern-most tip of South America, towards Mar del Plata.