Hopes have been dashed that sounds detected by two Navy ships may have come from a missing Argentinian submarine.
The ARA San Juan submarine, which has been missing since Wednesday with 44 crew on board, disappeared 430km off the Argentine coast and no trace of it has been found.
The navy said it reported a mechanical breakdown in its last communication.'
"The vessel surfaced and it reported a breakdown," naval commander Gabriel Galeazzi said.
Capt Galeazzi spoke of a "short circuit" in the sub's batteries.
This is the first time that an official has mentioned the sub encountering mechanical problems.
Earlier today, two Navy ships picked up noises on sonar probes which reportedly sounded like metal tools being banged against the submarine's hull.
But the Argentinian Navy has confirmed the sounds did not come from the missing sub.
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.
"A warship has a lot of backup systems, to allow it to move from one to another when there is a breakdown," he said.
The naval commander said that the submarine had been asked to cut short its mission, which was originally due to last until Monday, 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.
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 crew members were alive.
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.