Argentina's navy says it's trying to make a visual inspection of another object that registered on a sonar search for remains of a submarine that vanished 18 days ago with 44 crew members aboard.

Navy spokesman Enrique Balbi told a news conference on Sunday that the new object was detected at a depth of 3,100 feet (950 metres).

Balbi earlier said that inspection of another object by a Russian submersible revealed it was the wreck of a fishing vessel.

"It was confirmed that it wasn't the submarine's shell" but a sunken fishing vessel, Balbi said.

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The navy "regrets that without a rigorous analysis expectations were generated in the families and society, which must now face another frustration," Balbi said.

The search is taking place near the last known location of the ARA San Juan before it went silent off the Atlantic coast on November 15.

The navy said on Thursday that it is no longer looking for survivors, although a multinational operation will continue searching for the vessel.

In this September 27, 2011 photo, workers stand around the ARA San Juan submarine. Photo / AP
In this September 27, 2011 photo, workers stand around the ARA San Juan submarine. Photo / AP

"The extreme environment, the time elapsed and the lack of any evidence eliminates a scenario compatible with human life," Balbi said. "These are hours of intense pain and anguish (for relatives) in light of the loss of their loved ones, our 44 comrades."

The German-made submarine went missing as it was journeying from the extreme southern port of Ushuaia to the city of Mar del Plata, about 400 kilometres southeast of Buenos Aires.

The navy has said the vessel's captain reported that water entered the snorkel and caused one of the submarine's batteries to short circuit. An explosion was later detected around the time and place where the San Juan last made contact.

On Thursday last week, authorities formally ended the search for survivors, shifting its mission from rescue to recovery.

Relatives of crew members of the missing ARA San Juan submarine stand outside the navy base in Mar del Plata, Argentina. Photo / AP
Relatives of crew members of the missing ARA San Juan submarine stand outside the navy base in Mar del Plata, Argentina. Photo / AP

"The extreme environment, the time elapsed, and the lack of any evidence prevent sustaining a scenario compatible with human life," Balbi said.

In its final communication, the submarine reported it had overcome a mechanical breakdown that resulted from a short circuit due to the entry of water via the vessel's snorkel.

Three hours later, a noise similar to an explosion was recorded 48 kilometres from where the crew had given its last report.

The position was in line with the planned path the submarine would have taken to reach its base in Mar del Plata, the navy has said.

An international armada of rescue ships backed by aircraft — and thousands of personnel — has been hunting for the submarine.