Speaking at a forum on open government in Buenos Aires, Argentina's President Mauricio Macri said that "in these hard hours, I did not want to start without sending once again all my support to the families of the crew of the ARA San Juan submarine".
Hopes for a successful search for the submarine waned when the navy said satellite calls detected over the weekend did not in fact come from the vessel.
In the "worst-case scenario", the missing sub could run out of oxygen in two days, said Argentine navy spokesman Enrique Balbi.
Navy spokesman Enrique Balbi says Monday that a USA aircraft was sent to check an area where the noise was heard by two Argentine Navy ships.
"The submarine surfaced and reported a malfunction, which is why its ground command ordered it to return to its naval base at Mar del Plata", he said.
The navy lost contact with the ARA San Juan submarine on November 15, shortly after the vessel's captain reported a failure in the battery system while the sub was submerged off Argentina's South Atlantic coast, the military has said.
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The submarine reportedly carried enough food, oxygen and fuel for the crew to survive about 90 days on the sea's surface, but only enough oxygen to last seven days underwater, he said.
The submarine was en route from Ushuaia, the world's southernmost city, to its base in Mar del Plata and was about 300 miles (480 km) off the coast when it gave its last location on Wednesday, soon after reporting an electrical malfunction.
This type of problem is considered routine, and the crew was reported safe, he added.
The Bluefin 12D, one of the unmanned underwater vehicles being used, is capable of conducting search operations at a maximum depth of nearly 5,000 feet for 30 hours. When the sub last made contact on Wednesday, five days ago, it was immersed, Balbi said. But experts later determined that neither was from the missing sub. The Argentine navy said an electrical outage on the diesel-electric-propelled vessel might have downed its communications. "This is why we are deploying all resources with high-tech sensors".
The San Juan carries red and green flares, Balbi said, but authorities will still try to identify the origin of the white signals. "They could not help determine a point on the map to help the search".
The US official said that the waters of the Atlantic Ocean where the sounds originated are extremely deep. More than a dozen boats and planes from Argentina, the United States, Britain, Chile and Brazil have joined the search for the vessel.
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