Hopes for a successful search for the ARA San Juan submarine, which went missing last Wednesday off the Argentinian coast, waned on Monday when the Navy said satellite calls detected over the weekend did not in fact come from the vessel.
The signals were received at 10:52 am (1352 GMT) and 3:42 pm (1842 GMT), but they did not lock in, thus preventing a full connection.
The U.S. Navy said early on Sunday morning that it would send an aircraft with 21 personnel from Jacksonville, Florida, to assist with the search.
There is a feeling of "cautious enthusiasm", naval expert Fernando Morales told C5N television.
A US Navy official familiar with the search cautioned that it was unclear whether the Argentines described the sound as something similar to tools being banged against the hull of a submarine as was previously reported.
Monday's revelations were a blow to relatives of sailors aboard the sub, around 100 of whom are being housed at the Mar del Plata naval base as they await news of the crew.
The communication attempts "indicate that the crew is trying to re-establish contact, so we are working to locate the source of the emissions", the Navy said on its Twitter account, adding that the calls lasted between four and 36 seconds.
Although the German-built diesel-electric vessel carried enough food, oxygen and fuel for the crew to survive about 90 days on the sea's surface, the sub had only have enough oxygen to last seven days submerged, Balbi said.
Rescuers are focusing on an ocean patch about 300 kilometers in diameter, radiating from the last point of contact. An unmanned underwater vehicle, it is capable of searching at a maximum depth of 5,000 feet for 30 hours. A Navy inquiry said the cause could not be definitively determined. The location of the noise coincides with the route the submarine would have taken on the way to its home port in Mar del Plata.
Argentine President Mauricio Macri met with family members at the base.
Claudio Rodriguez, whose brother Hernan is aboard the submarine, was hopeful, saying the satellite signals suggested the vessel was still afloat and would be found. "Thank God", he said.
The navy has not ruled out any hypothesis.
It is one of three submarines in the Argentine fleet.
"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.
It comes after it was revealed the submarine had reported a mechanical breakdown in its final communication.
SOUTHCOM directed the deployment of the P-8A aircraft, underwater rescue teams and personnel to assist Argentina as support, the country request for worldwide assistance to locate the missing submarine and crew.