An object detected by a US Navy plane near the area where a missing Argentinian submarine sent its last signal is "not the lost sub", officials said.

A witness on board the search plane earlier said an item had been spotted but the crew emphasised that it was not known if it was connected to the vessel.

A spokeswoman for the US embassy in Argentina later confirmed there was no link to the missing vessel.

The agonising wait for news of the whereabouts of the ARA San Juan has intensified in recent days as fears grow that the oxygen supply on board could be running low.

If the German-built submarine, in service for more than three decades, had sunk or was otherwise unable to rise to the surface since it gave its last location on 15 November, it would be using up the last of its seven-day oxygen supply.

The search for the submarine and its 44 crew has entered a "critical phase", Argentine navy spokesman Enrique Balbi said, as teams continue to scour 185,000 square miles of ocean - roughly the size of Spain.

Reports on Wednesday that a US Navy vessel had detected a "heat stain" and infrasound in the area were dismissed by Pentagon officials, who told The Independent it remained an active search operation.

The Argentine navy said it has lost contact with the the submarine off the country's southern coast (EPA)

A US Navy Boeing P-8A Poseidon jet is involved in the search alongside an RAF Voyager carrying equipment and submarine specialists.

The helicopter is packed with equipment, including 12 emergency life support pods. It joins HMS Protector, a Royal Navy ice patrol ship; the HMS Clyde, an offshore patrol vessel; and an RAF C-130 in the search.

The US aircraft has returned to its base in Bahia Blanca, around 200 miles south of the capital, Buenos Aires.