A sound detected in the hunt for a missing submarine is "consistent with a non-nuclear explosion", Argentina has said.

The ARA San Juan has been missing in the South Atlantic since the middle of November and dozens of planes and boats have been involved in search.

Some 44 people are on board and the operation has entered a "critical phase" because the crew's oxygen supply could be running low.

Navy spokesman Enrique Balbi said relatives of the crew members have been told about the development and the search will continue until the fate of the crew is known.

He described the blast, which was detected around the same time that the submarine sent its last signal last week, as "abnormal, singular, short, violent" and "non-nuclear".

Mr Balbi added there was no sign the explosion was linked to any attack on the vessel.

More than a dozen ships and planes are involved in the multinational search, with search teams combing an area of some 185,000 square miles, roughly the size of Spain.

Earlier, the United States said an object detected by its navy close to where the submarine sent its last signal was not the missing sub.

A P-8A Poseidon plane made the discovery - but analysis later ruled the object out as being the submarine.

The search is taking on an increasing sense of urgency as experts worry that if the ARA San Juan is intact but submerged, its crew might only have enough oxygen to last another seven to 10 days.

The crew could survive indefinitely if the 34-year-old German-built submarine is able to reach the surface periodically to replenish its air.

Relatives of the crew continue to wait at a naval base in Mar del Plata, as the case grips Argentina.

The hashtags "Los 44" (The 44) and Enrique Balbi have become trending topics on Twitter and relatives have featured on the front of newspapers in the country.

The submarine had been travelling from Ushuaia, the world's southernmost city, to Mar del Plata when it reported an electrical problem just before it vanished.