Authorities were investigating white flares spotted in the south Atlantic overnight, as urgency grew in the search for a missing Argentine navy submarine carrying 44 crew members.

Concern is growing that the submarine’s crew could be running out of oxygen.

Searchers found an empty raft floating in the ocean, and noticed white flares from a distance.

But the raft's brand suggested it did not belong to the ARA San Juan, which was equipped with only red flares for emergencies and green flares for other situations, a navy official said.

Still, boats searching for the submarine were heading toward the flares to investigate further, a navy spokesman said.

The submarine was en route from Ushuaia, the world's southernmost city, to its base in Mar del Plata and was about 480km off the coast when it gave its last location last Wednesday, soon after reporting an electrical malfunction.

If the German-built vessel had sunk or was otherwise unable to rise to the surface since it sent its last signal, it would be winding down its seven-day oxygen supply.

"Assuming the worst, that it was underwater and could not snorkel - which means renewing air and oxygen - and could not rise to the surface on its own, we would be in the sixth day of oxygen," the navy spokesperson told reporters.

More than a dozen boats and planes from Argentina, the United States, Britain, Chile and Brazil have joined the search for the vessel. Authorities have mainly been scanning from the sky as storms have halted the maritime hunt.

The weather was beginning to improve this afternoon as wind speeds slowed and waves that rose as high as 26ft over the weekend diminished.

Searchers have suffered disappointments in recent days as analysis of satellite signals and sounds detected by underwater probes - initially thought to be messages from the crew – has found they did not come from the vessel.

"Today is a critical day," said a mother of one of the missing crew members. "We are holding up as well as we can."

Other relatives of crew members have gathered at a naval base in Mar del Plata, where authorities are coordinating the search and rescue operation.

The US Navy was preparing to deploy rescue equipment, including a remote-operated vehicle.