The search for an Argentine navy submarine missing in the South Atlantic for a week has reached a "critical phase" as the 44 crew on board could be running low on oxygen, a navy spokesman said.

Dozens of planes and boats were searching for the ARA San Juan, a mission that has plunged relatives of the sailors into an anguished wait for news and transfixed the South American country.

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.

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

"There has been no contact with anything that could be the San Juan submarine."

Favorable weather meant search boats could cover a greater area after being hampered by strong winds and high waves fo rmuch of the past few days, although poor weather was expected to return tomorrow, Mr Balbi said.

Around 30 boats and planes and 4,000 people from Argentina, the United States, Britain, Chile and Brazil have joined the search for the submarine, which last transmitted its location about 480km from the coast.

Planes have covered some 500,000 square km of the ocean surface, but much of the area has not yet been scoured by the boats.

As part of the search operation, a P-8 Poseidon airplane from the US Navy left the Comandante Espora base some 650km south of Buenos Aires to try to detect the submarine through "sonobuoys," or buoys equipped with a sonar system that are dropped from aircraft, usually during anti-submarine warfare.

In recent days several possible signals, including sounds and flares, that have been detected have turned out to be false alarms.

Overnight, a British ship reported observing three orange and white flares, but they did not come from the vessel, Mr Balbi said.

Relatives of the crew members have been gathered at a naval base in Mar del Plata, where the search is being coordinated.

"We came today because we had hope that they had returned," Elena Alfaro, a sister of a crew member said in tears.

"It is incomprehensible that so much time has passed. We are in pain."