The Argentine Navy has warned that oxygen supplies on board could be running low
The search for a missing Argentine Navy submarine has entered a "critical phase" as the crew's oxygen supply could be running low.
Dozens of planes and boats are searching for the ARA San Juan, which has been missing in the South Atlantic since November 15th with 44 crew on board.
The submarine has only seven days of oxygen supply and, if it has sunk or been unable to surface since it was last heard from, then it could be using the last of its supply.
Navy spokesman Enrique Balbi said: "We are in the critical phase... particularly with respect to oxygen.
"There has been no contact with anything that could be the San Juan submarine.
"We are in a very dangerous situation and one that is getting worse."
As long as the 34-year-old, German-built, submarine can reach the surface periodically to replenish its air, the crew could survive indefinitely – however, there is nothing to indicate they have achieved this.
Naval investigator Fernando Morales told Reuters: "If the captain stayed at the bottom because he thought it was more prudent to stay at the bottom, it's one thing.
"But at this point we have to think that if he's at the bottom, it's because he could not emerge."
After a few days of good weather, searchers - including 30 boats and planes and personnel from the US, UK, Chile and Brazil - are expecting deteriorating conditions from Thursday.
Mr Balbi revealed that a noise had been detected almost three hours after the submarine disappeared, near its last reported position.
He described this as a "hydro-acoustic anomaly" but declined to say whether it indicated an explosion or emergency on the submarine.
The noise is being analysed, he said, adding that it had to be "corroborated and looked into."
Several possible signals, including sounds and flares, that were previously detected in the search turned out to be false alarms.
Meanwhile, relatives of the crew member 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 newspaper front pages.
ARA San Juan had been travelling from Ushuaia, the world's southernmost city, to Mar del Plata when it reported an electrical problem just before it vanished.
The incident has brought criticism from some, including relatives of those missing, of the ageing equipment used by Argentina's armed forces.