The search for a missing Argentinian 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 15 November with 44 crew on board.

But the submarine has only seven days of oxygen 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.

Image: The Argentinian navy is taking part in the hunt for the ARA San Juan submarine

Search teams are combing a wide area of 185,000 square miles, which is about the size of Spain.

Navy spokesman Enrique Balbi said: "We are in the critical phase... particularly with respect to oxygen.


Image: It has now been more than a week since ARA San Juan went missing

"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, although there is nothing to indicate they have achieved this.

Image: The Argentine military submarine ARA San Juan was 34 years old

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.

Image: Enrique Balbi says it is a 'very dangerous situation and one that is getting worse'

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.

Image: Apprehensive relatives of those missing are waiting at a naval base for news

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.