Updated at 4:10 a.m. ET

Stormy conditions off the coast of Patagonia were hampering efforts to locate a missing Argentine submarine with 44 crew members. Doubts also surfaced over the origin of satellite signals that were initially thought to have come from the vessel.

Communications with the ARA San Juan, a German-built diesel-electric submarine that entered service in the Argentine Navy in 1985, were lost on Wednesday. The vessel was returning to the Mar del Plata Naval Base south of Buenos Aires at the conclusion of a routine patrol to the far southern port of Ushuaia.

Waves up to 20-feet in the area where the sub went missing, about 260 miles from the Argentine coast, were complicating the international search effort, Adm. Gabriel Gonzalez, commander of the base, was quoted by The Associated Press as saying. He said similar conditions were expected for the next two days.

On Saturday, three days after losing contact with the San Juan, officials said they had received seven satellite signals that they hoped would help in pinpointing its location. However, by Sunday those hopes appeared to have been dashed.

"We do not have clear evidence that (the calls) have come from that unit," Gonzalez said. "We are analyzing more closely to reliably determine that they were not calls coming from the submarine."

The New York Times reports that Iridium, the satellite phone company, said it had "found no evidence that an Iridium phone aboard the vessel had been used since Wednesday morning." Reuters adds that Iridium says the last call detected came on Wednesday, the same day the submarine went silent.

Navy spokesman Enrique Balbi, quoted by the AP, said the low-frequency satellite signals received on Saturday lasted only a "few seconds," but were initially thought to have been attempts by the crew to re-establish contact.

The BBC reports that "It is thought that the submarine may have had communication difficulties caused by a power cut. Navy protocol dictates that a vessel should come to the surface if communication has been lost."

According to Gonzalez, more than a dozen ships from Argentina, the United States, Britain, Chile and Brazil were involved in the search, but that the rough weather had mainly limited the operation to aerial reconnaissance.

Two U.S. aircraft carriers were part of the search team.

NPR's Philip Reeves reports that officials say the crew should have enough food and oxygen aboard. He says the U.S. Navy was dispatching assistance from its Undersea Rescue Command headquartered in San Diego.

In a statement issued late Sunday, the U.S. Naval Forces Southern Command said it had deployed unmanned underwater vehicles (UUVs) to join the search.

"The equipment consists of one Bluefin 12D (Deep) UUV and three Iver 580 UUVs, which are operated by the U.S. Navy's recently-established Unmanned Undersea Vehicle Squadron 1, based in Pearl Harbor, Hawaii," the statement said. "The UUVs are uniquely capable to help in the search. Both types are capable of deploying quickly and searching wide areas of the ocean using Side Scan Sonar, a system that is used to efficiently create an image of large areas of the sea floor."

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