The ARA San Juan made its last contact with authorities on November 15 to report a mechanical breakdown, but, as storm conditions which had impeded rescuers eased, officials said they were not ceding to despair yet.
"We haven't discarded any hypothesis. Assuming the most critical phase, which would be the submarine is submerged and cannot surface, we are in the 6th day of oxygen," navy spokesman Enrique Balbi told a news conference in Buenos Aires.
The vessel would have enough oxygen for its crew to survive underwater for seven days, if there was no hull breach, according to officials.
"As a submariner, I am not losing hope," Balbi said.
The 34-year-old German-built diesel-electric submarine that was refitted between 2007 and 2014, had notified a breakdown in its batteries and said it was diverting its route from the far south of Argentina's Atlantic waters to the navy base at Mar del Plata, where most of the crew live.
It didn't issue a distress call, however. It was not known if the problem ended leaving the vessel without propulsion or unable to surface.
The search began November 16, when there was no more contact.
National apprehension
The sub's disappearance has gripped the nation, and President Mauricio Macri visited the relatives and prayed with them.
On Monday, underwater sounds detected briefly caused hopes to surge, but they were later ruled out as not coming from the sub.
"It may have been a noise from a living thing," Balbi said on Monday.
Several satellite signals had also caused a start over the weekend, but they too were dismissed as not coming from the vessel.
"A little glimmer had started to shine and then it went out," Maria Morales, the mother of one of the sailors, said as she arrived at the Mar del Plata base on Tuesday.
Argentina is leading an air-and-sea search with help from nine countries, including Brazil, Britain, Chile, Colombia, France, Germany, Peru, the United States and Uruguay.
Poor weather in the South Atlantic had badly hampered the operation in the past few days. But Tuesday offered better conditions, with strong winds and waves that had towered as high as six meters (20 feet) starting to calm.
"Luckily, wind intensity has started to drop, and waves are three to four meters, which should allow a three-dimensional sweep of the (sea) floor," Balbi said.
The initial search zone was already big, with a diameter of 300 kilometers (200 miles) and a depth of up to 350 meters (1,200 feet). But there is a possibility it could be expanded, to seven times the area, according to the defense ministry.
'They will come back'
The incident has recalled recent submarine disasters, perhaps most prominently that of the Kursk, a Russian nuclear sub that caught fire and exploded underwater in 2000, killing all 118 on board -- some instantly, others over days.
There was also a Chinese sub accident in 2003 that killed 70 crew, apparently suffocated after what Beijing termed "mechanical problems."
Among the ARA San Juan's crew is Argentina's first female navy submariner: Eliana Krawczyk, 35.
Cards, banners with slogans and placards have been strung up on the outside of the Mar del Plata base's wire fence, expressing solidarity with the families tensely waiting for any news.
"There's a mix of feelings: pain, helplessness, at times hope," Maria Morales said. "The feeling is that they will come back, that we will tell us today, 'They are back.'"
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