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 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. "The calls of a short duration, between 4 and 36 seconds, were received between 10:52 and 10:42 on Saturday at different bases."
But Argentine authorities cautioned that it has not been confirmed the calls came from the submarine, though that is the working hypothesis.
The sub's crew includes Argentina's first female submarine officer, 35-year-old Eliana Krawczyk, Agence France-Presse reported.
The U.S. Navy announced late Saturday it was sending a P-8 Poseidon reconnaissance plane with a crew of 21 will depart from Jacksonville, Fla., on Sunday, according to a statement from the U.S. Southern Command. The P-8 will join a NASA P-3 plane and another Navy P-8 already taking part in the search.
BREAKING: @USNavy deploying additional P-8A Poseidon to Argentina to support the ongoing international search for the Argentinean Navy’s submarine A.R.A. San Juan. (U.S. Navy file photo) https://t.co/9erSLA0NxX pic.twitter.com/1FGZFY6qaX
— US Southern Command (@southcomwatch) November 19, 2017
The Navy has also sent two undersea rescue systems to Argentina aboard four U.S. Air Force cargo planes.
Adm. Gabriel Gonzalez, chief of the Mar del Plata Naval Base, said personnel there were 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.
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.
The satellite communications were believed to have failed because of foul weather, a source in the defense ministry who was not authorized to speak publicly told Reuters.
The search and rescue mission has so far been hampered by adverse weather conditions, including high winds and 20-foot waves in the South Atlantic.
Slightly better weather was predicted for Sunday, Argentine navy spokesman Enrique Balbi said.
On Saturday, 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.
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.
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, which sent a NASA scientific aircraft and a Navy plane. Britain was sending a polar exploration vessel, the HMS Protector, which British officials said should arrive Sunday.
From the Vatican, Argentine Pope Francis said he was making "fervent prayers" for the crew.