The search for a missing Argentine submarine and its 44-member crew has been helped by calmer seas, but there were no new clues about its location and worries multiplied because the vessel may be running out of air.

While the ARA San Juan carried enough food, oxygen and fuel for the crew to survive about 90 days on the sea’s surface, it had only enough oxygen to last seven days if submerged.

The San Juan was last heard from last Wednesday as it journeyed from the extreme southern port of Ushuaia to the coastal city of Mar del Plata.

More than a dozen international vessels and aircraft have joined in a maritime search that has become a race against the clock.

“The most critical situation would be that we are in the sixth day of oxygen,” the navy’s spokesman, Captain Enrique Balbi, said at a news conference.

“Obviously that is worrisome. Each time we are entering little by little a critical period.”

A US aircraft searching for the missing submarine spotted white flares, but the Argentine navy said they were unlikely to be from the San Juan.

The vessel carried red and green flares, but authorities would still try to identify the origin of the white signals, navy spokesman Enrique Balbi told reporters.

“We’re evaluating where the flares came from. For now, based on the colour, they don’t belong to the submarine,” Mr Balbi said.

An Argentine navy official previously said the submarine reported a battery failure last Wednesday and was returning to the Mar del Plata naval base when it went missing.

Hopes were buoyed after brief satellite calls were received and when sounds were detected deep in the South Atlantic, however experts later determined that neither was from the missing sub.

Map of Argentina’s San Jorge Gulf, the submarine’s last known location.

Locals hold vigil near naval base

The sub was originally scheduled to arrive on Monday at Mar del Plata, about 400 kilometres south-east of Buenos Aires.

Relatives of the crew have gathered at the base to receive psychological counselling and wait for news about their loved ones.

Some local residents gathered outside the base to pray on Tuesday, gripping rosary beads and religious statuettes.

Others hung pictures of the crew on a fence, where children from a local school had placed drawings and messages of support for the families of the missing sailors.

The US Navy has sent its Undersea Rescue Command to Argentina to support the search for the submarine.

The command includes a remotely operated vehicle and vessels that are capable of rescuing people from bottomed submarines.

The crew members aboard the San Juan include Argentina’s first female submarine officer.