An international nuclear test-ban body that runs a global network of listening posts designed to check for secret atomic blasts detected an "unusual signal" underwater last week near where an Argentine submarine went missing, it said on Thursday.
The Vienna-based Comprehensive nuclear Test-Ban Treaty Organization (CTBTO) runs monitoring stations equipped with devices including hydrophones - underwater microphones that scan the oceans for sound waves.
Two of its monitoring stations detected the unusual signal, the CTBTO said in a statement, adding that it was passing the information on to the Argentine authorities coordinating the search for the ARA San Juan, which had 44 crew on board when it went missing last week.
An Argentine navy spokesman said earlier on Thursday that an abnormal sound detected near the submarine's last known position in the South Atlantic was "consistent with an explosion", but the CTBTO was more guarded on the possible cause.
"It could be consistent with an explosion but there is no certainty about this," CTBTO hydroacoustic engineer Mario Zampolli told Reuters. He agreed with the navy spokesman's description of the signal as unusual and short, adding that the cause was non-natural.
"We can also calculate the time when the event happened in that location and that time is about three and a half hours after the last contact that the sub apparently had, according to what's out in the news," he said. The stations that detected it are far apart, in the Atlantic and Indian oceans, he added.
Underwater sounds detected in the first days of the search by two Argentine search ships were determined to originate from a sea creature, not the vessel. Satellite signals were also determined to be false alarms.
Russia was the latest navy to volunteer to a multinational sea search, sending an oceanographic research ship as the operation shifted focus from rescue to recovery.
The Russian defense ministry said the Yantar was steaming to the area from the western coast of Africa on the orders of President Vladimir Putin.
The weeklong search has focused on the sub's last known position, around 200 miles (320 kilometers) off the Argentine coast, but has been hampered by bad weather.
The San Juan, a 34-year-old German-built diesel-electric submarine, had reported a battery problem on November 15 and said it was diverting to its home base at Mar del Plata, but did not send a distress signal, according to the navy.
Argentina is leading an air-and-sea search with help from several countries now including Brazil, Britain, Chile, Colombia, France, Germany, Peru, Russia, the United States and Uruguay.