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Malvina Vallejos, the sister of missing submariner Celso Oscar Vallejos, hangs a message for the 44 crew members of the Argentine submarine outside Argentina’s Navy base in Mar del Plata, Buenos Aires, on Tuesday. An international search mission for the missing Argentine submarine entered its sixth day Tuesday as uncertainty over the fate of its 44 crew members gave way to rising anguish for families troubled by earlier false hopes. Photo: AFP

The search for an Argentine navy submarine missing in the South Atlantic for one week reached a "critical phase" on Wednesday as the 44 crew members on board could be running low on oxygen, a navy spokesperson 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 of 44 million people.

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 November 15, 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 spokesperson Enrique Balbi told reporters. "There has been no contact with anything that could be the San Juan submarine."

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

The craft was probably on the seabed because the mechanism to surface either failed or was not activated by a crew member, naval investigator Fernando Morales told Reuters in a telephone interview.

"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," Morales said.

In an evening news conference, Balbi said an unusual noise was detected on November 15, near where the submarine last reported its position. He declined to say if the sound indicated an explosion or emergency on the vessel. Data on the noise were being analyzed, he added.

Favorable weather allowed search boats to cover a greater area after being hampered by strong winds and waves for much of the past few days, Balbi said. Poor weather was expected to return on Thursday.

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