The US says an object its navy detected near to where an Argentinian submarine sent its last signal is not the missing vessel.

:: LATEST - Sound is 'consistent with explosion'

A P-8A Poseidon plane made the discovery but analysis later ruled the object out as being the ARA San Juan, said the American embassy.

Some 44 people are on board the missing sub and the search is in 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.


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: A major search is taking place for the submarine

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.

After a few days of good weather, searchers - including 30 boats and planes and personnel from the US, UK, Chile and Brazil - were expecting deteriorating conditions from Thursday.

Navy spokesman Enrique 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: Members of the Argentinian Navy are pictured in the air looking in the waters below for the vessel

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.

The 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.