MAR DEL PLATA, Argentina/BUENOS AIRES (Reuters) - 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 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 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 Nov. 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 spokesman 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. Their concern grew as the hours ticked by. 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 Nov. 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 United States, Britain, Chile and Brazil have joined the search for the submarine, which last transmitted its location about 480 km (300 miles) from the coast.  Planes have covered some 500,000 square km (190,000 square miles) of the ocean surface, but much of the area has not yet been scoured by the boats. Argentines have been gripped by the search, with local newspapers placing photographs on their front pages of crew members  relatives praying. The case has dominated discussion on social media in Argentina, with the hash tags  Los 44  (The 44) and (navy spokesman)  Enrique Balbi  becoming trending topics on Twitter. Comparisons were made to the most recent major rescue operation in the region, when 33 miners in northern Chile were rescued in 2010 after 69 days trapped underground. The submarine was en route from Ushuaia, the southernmost city in the world, to the coastal city of Mar del Plata, some 400 km (250 miles) south of Buenos Aires, when it reported an electrical malfunction shortly before disappearing last week. The submarine was launched in 1983 and underwent maintenance in 2008 in Argentina. The disappearance has highlighted the dwindling resources and lack of training faced by the armed forces since the end of a military dictatorship in the early 1980s. 