There had been no contact with the ARA San Juan since early Wednesday, according to the navy, prompting Buenos Aires to launch an air and sea search with help from countries including Brazil, Britain, Chile and the United States.
The entire search area has been scoured by ships and aircraft, despite storm conditions that complicated the effort, Argentine navy spokesman Enrique Balbi said.
Yet finally seven satellite-transmitted signals believed to be part of the vessel trying to resume contact were detected, the Defense Ministry said.
With the help of US satellite communication experts, the signals were detected at 10:52 am (1352 GMT) and 3:42 pm (1842 GMT) on various naval bases, but did not lock in, thus preventing a full connection.
"Right now, we are working to pinpoint the exact location of what is emitting the signals," presuming that it could be the missing sub, the ministry said.
Brazil, Britain, Chile, the United States and Uruguay took part in the aerial side of the search, and Washington said it was sending rescue help.
The California-based Undersea Rescue Command was deploying two independent rescue assets -- including a pressurized rescue module -- to help in the hunt for the missing sub.
'Got to be afloat'
Argentine President Mauricio Macri said on his Twitter account that "we will do what is necessary to find the submarine as soon as possible."
Claudio Rodriguez, whose brother Hernan is aboard, was hopeful and thought the vessel would be found afloat, if the satellite signals were able to be sent.
"They've got to be afloat. Thanks God," Rodriguez stressed.
"That gives us hope, because we knew that if they were down below, they would be screwed" he told TN news from Mendoza.
The navy has not ruled out any hypothesis, a spokesman said. The most commonly given is that a power short may have unexpectedly cut off the vessel's communications.
TR-1700 class diesel electric submarine had been returning from a routine mission to Ushuaia near the southernmost tip of South America, to its base at Mar del Plata, around 400 kilometers (250 miles) south of Buenos Aires.
Among those on board is Argentina's first female submarine officer, 35-year-old weapons officer Eliana Krawczyk.
The San Juan is one of three submarines in the Argentine fleet.
Sixty-five meters (213 feet) long and seven meters (23 feet) wide, it was built by Germany's Thyssen Nordseewerke and launched in 1983.
It underwent a re-fit between 2007 and 2014 to extend its usefulness by some 30 years.
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