A desperate search has intensified for 44 crew members of an Argentine navy submarine that went missing in the South Atlantic.
Whipping winds and 20-foot waves have hampered the hunt for the German-built ARA San Juan, which was 268 miles off Argentina's southern Atlantic coast when it sent its last known communication on Wednesday.
Search units have relied on information gathered from a British polar exploration vessel, the HMS Protector, which was equipped with an underwater search probe and was following the path taken by the submarine.
"Our thoughts remain with the crew of the ARA San Juan and their families at this time," said HMS Protector Commander Angus Essenhigh, according to a statement from the Royal Navy.
The US Navy's Undersea Rescue Command has also been deployed to the search area, along with aircraft from Argentina, Brazil and the U.S., and 11 surface vessels.
Seven failed satellite calls possibly made from the vessel raised hopes the crew are alive, but the Argentine navy has said it cannot yet confirm where they came from.
The calls were made 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 unnamed US company specialised in satellite communication to trace them.
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.