By Marcos Brindicci and Luc Cohen
MAR DEL PLATA, Argentina/BUENOS AIRES (Reuters) - A search and rescue operation for an Argentine navy submarine missing in the South Atlantic with 44 crew members aboard reached its third day on Sunday, after failed satellite calls likely from the vessel raised hopes the crew are alive.
The U.S. 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.
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 U.S. company specialized in satellite communication to trace the location.
"Yesterday's news was something of a respite for us, to know that there is life," Claudio Rodriguez, the brother of a crew member, said in an interview with television channel A24 on Sunday morning.
The new U.S. navy aircraft would join another U.S. aircraft and Argentine planes and sea vessels scouring the southern sea as whipping winds and more than 20-foot waves hindered the search. Nations from Chile to Britain and South Africa also offered help.
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 Argentine navy spokesman Enrique 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.
Family members of the crew gathered at a naval base in the coastal city of Mar del Plata, where the submarine had been destined to arrive.
Argentine-born Pope Francis mentioned the missing vessel in his Sunday noon prayer.
"I also pray for the men of the crew of the Argentine military submarine which is missing," the pontiff said.
The dramatic search has captivated the nation of 44 million, which recently mourned the loss of five citizens killed when a truck driver plowed through a bicycle path in New York City.
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.
(Additional reporting by Keith Coffman in Washington, D.C. and Philip Pullela in Rome; Editing by Mark Potter)