Argentina’s navy is investigating reports of a loud noise detected a few hours after a submarine went missing.
The ARA San Juan disappeared last Wednesday with 44 people on board.
A spokesman called the noise a “hydro-acoustic anomaly” and would not confirm whether there had been an explosion.
Captain Enrique Balbi said the situation was getting critical and “concern is growing more and more” amid fears oxygen on board is close to running out.
One crew member’s sister said: “I feel like I’m waiting for a corpse.”
Elena Alfaro, who was waiting alongside other family members at the Mar del Plata naval base, told TN news network: “I feel like I’m at a wake. I also feel time passing and time is crucial.”
More than a dozen countries including Russia and the US have sent resources to take part in the search for the submarine.
On Thursday, a US Navy aircraft detected an “object” near the submarine’s last known location, the news agency Reuters reported, but the US Navy quickly confirmed that it was not from the missing sub.
On Wednesday, Mr Balbi told journalists: “Today is the seventh day, which was critical for oxygen capacity.”
The loud noise happened four or five hours after the submarine’s last radio contact, about 30 nautical miles (60 kilometres) north of its last-known position.
“It’s a noise,” Mr Balbi said. “We don’t want to speculate.”
Brazil, Chile, Colombia, France, Germany, Peru, South Africa, Uruguay and the UK are among the countries that have sent either ships or planes to help with the search.
The US navy has deployed two underwater vehicles which use sonar to create images of the sea floor.
A Nasa research aircraft has also flown over the search area but failed to spot anything.