The US Southern Command said Sunday morning it's sending a Submarine Rescue Chamber, designed during WWII, which can reach a submarine submerged up to 850 feet, and bring up to six people at a time back to the surface.
Argentina's Navy said it detected seven brief satellite calls late Saturday that officials believe may have come from the ARA San Juan, which hasn't been heard from since Wednesday.
The UK, US, Brazil, Chile and Uruguay have joined the search operation for the submarine, which disappeared 268 miles off Argentina's southern Atlantic coast on Wednesday.
"We do not have clear evidence that (the calls) have come from that unit", said Adm. Gabriel Gonzalez, commander of the Mar del Plata Naval Base.
Following a request for assistance from the Argentine government, the Portsmouth-based navy ship immediately changed course from her planned tasking and sailed to join the multi-national efforts to find the missing submarine ARA San Juan. The aircraft are outfitted with instruments that detect anomalies above and below the ocean surface.
Britain is sending a polar exploration vessel, the HMS Protector, which officials said should arrive on Sunday. A U.S. company that specializes in satellite communication is trying to help the Argentines pinpoint the location of the vanished sub.
Relatives of the crew members gathered at the Mar del Plata Naval Base in the hopes of hearing news about their loved ones. Among them was Ester Alfaro, mother of crew member Cristian Ibanez.
News of the stricken submarine had even reached the Vatican.
Special correspondents D'Alessandro reported from Buenos Aires and Kraul from Bogota, Colombia, respectively.
The navy said an electrical outage on the diesel-electric-propelled vessel might have downed its communications. Francis "asks that his closeness be conveyed to their families and to the military and civil authorities of the country in these difficult moments".
The most likely explanation was that fuel in a torpedo detonated, setting off a chain reaction in a sub once deemed unsinkable.
Sixty-five meters long and seven meters wide, it was built by Germany's Thyssen Nordseewerke and launched in 1983.
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.
It is thought that the submarine may have had communication difficulties caused by a power cut. However, it underwent a seven-year refit between 2007 and 2014 to extend its life by a further 30 years.