Seven brief satellite calls were detected on Saturday by the Argentine Navy, raising speculation that they may have come from a submarine that has been missing for three days.
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.
The ARA San Juan went missing with 44 crew members.
Argentine authorities clarified that it has not been confirmed the calls came from the submarine, though that is the working hypothesis.
Authorities last had contact with the German-built, diesel-electric sub on Wednesday as it was on a voyage from the extreme southern port of Ushuaia to Mar del Plata.
Earlier Saturday, Navy spokesman Enrique Balbi said the area being searched off the country's southern Atlantic coast has been doubled as concerns about the fate of the submarine and its crew grew.
"We are not discounting any hypothesis," Balbi said, adding that possibilities to explain the submarine's disappearance include "a problem with communications" or with its power system.
Argentine President Mauricio Macri said in a tweet that the country will use "all resources national and international that are necessary to find the submarine."
Pledges of help came from Chile, Uruguay, Peru and Brazil, as well as the United States.
The US Navy said early Sunday morning it would send an aircraft with 21 personnel from Jacksonville, Florida, to assist with the search for the German-built ARA San Juan, which was 432 km (268 miles) off Argentina's southern Atlantic coast when it sent its last communication early on Wednesday.
A search of 80 percent of the area initially targeted for the operation turned up no sign of the vessel on the ocean surface, but the crew should have ample supplies of food and oxygen, according to Balbi.
The navy said an electrical outage on the diesel-electric-propelled vessel might have downed its communications. Protocol calls for submarines to surface if communication is lost.
Britain was sending a polar exploration vessel, the HMS Protector, which British officials said should arrive Sunday.
Admiral Gabriel Gonzalez, chief of the Mar del Plata Naval Base, said they are coordinating "with units from the United Kingdom and the United States." Britain and Argentina fought a war in 1982 over the Falklands Islands, which are called the Malvinas in Argentina.
The submarine likely tried to make seven satellite calls on Saturday between late morning and early afternoon, the Argentine defense ministry said. Stormy weather likely interfered with the calls, and the government was working with an unidentified US company specialized in satellite communication to trace the location.
Relatives of the crew members gathered at the Mar del Plata Naval Base in the hopes of hearing news about their loved ones.
"We feel anguish. We are reserved but will not lose our hope that they will return," Marcela Moyano, wife of machinist Hernan Rodriguez, told television network TN.
She said she spoke with her husband when the submarine departed and is still sending him WhatsApp messages, though he has not responded.
From the Vatican, Argentine Pope Francis said he was making "fervent prayers" for the crew.
The ARA San Juan was inaugurated in 1983, making it the newest of the three submarines in the navy's fleet. Built in Germany by Nordseewerke, it underwent mid-life maintenance in 2008 in Argentina.
