The Argentine navy says an event consistent with an explosion was recorded near where a submarine disappeared last week with 44 crew on board.
A navy spokesman said an “abnormal, singular, short, violent, non-nuclear event” had been detected in the south Atlantic.
The ARA San Juan disappeared last Wednesday.
More than a dozen countries including Russia and the US have sent assistance.
Spokesman Captain Enrique Balbi said the Argentine navy had been informed of the suspected explosion, which took place near the submarine’s last known location.
Captain Balbi said the navy only knew the location of the suspected explosion, not its cause.
Search efforts would be concentrated in the area, he said.
On Wednesday, Captain Balbi said oxygen would be running out on board the vessel.
Relatives have been waiting for news outside the Mar del Plata naval base.
One crew member’s sister said: “I feel like I’m waiting for a corpse.”
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.
What was the sub’s last known location?
The ARA San Juan was returning from a routine mission to Ushuaia, near the southern-most tip of South America, when it reported an “electrical breakdown”.
According to naval commander Gabriel Galeazzi, the submarine surfaced and reported the breakdown, which Capt Galeazzi described as a “short circuit” in the sub’s batteries.
The sub was ordered to cut its mission short and return to the naval base in Mar del Plata immediately.
According to Mr Balbi, the captain of the ARA San Juan contacted the naval base once more after reporting the problem.
In the message, he reportedly said that the problem had been adequately fixed and that the sub would submerge and proceed towards Mar del Plata naval base.
The last contact was made at 07:30 local time (10:30 GMT) on Wednesday 15 November. It is not known what happened to the sub after that contact.
How was the alarm raised?
Argentine navy protocol stipulates that in peace time, submarines make contact twice a day with the base.
When the submarine failed to call in, the Argentine navy began its search for the vessel.
Navy commander Carlos Zavalla at that point spoke only of a “failure to communicate” and urged relatives of the crew to remain calm.
There was no mention by navy officials of the vessel having any problems at the time and rumours of a fire on board were dismissed by Mr Balbi.
Who is on board?
There are 44 crew on board the submarine, which is under the command of Pedro Martín Fernández.
Forty-three of the crew are men but there is also one woman, Eliana María Krawczyk. The 35-year-old is the first female officer in Argentina to serve on a submarine.
Nicknamed “the queen of the sea” by her father she comes from Oberá, a city in northern Argentina.
Despite having been born and raised far inland, her relatives say that “she was born to be a submariner”, citing her “will of steel” and a passion for her job.
The rest of the crew is made up of submariners of varying ages and experience.
The sub’s engineer, Hernán Rodríguez, has been on the ARA San Juan for 11 years, local media reported.