Argentina’s navy said on Wednesday it was investigating an unusual noise detected in the South Atlantic hours after it last communicated with a missing submarine, but refused to confirm whether it indicated an explosion.

The development came as the clock was ticking down on hopes of finding alive the 44 crew members now missing for a week despite a massive search of surface and seabed, amid fears their oxygen had run out.

The ARA San Juan would have had enough oxygen for its crew to survive underwater in the South Atlantic for seven days since its last contact, according to officials. At 0730 GMT on Wednesday, that time had elapsed.

Navy spokesman Enrique Balbi told reporters a "hydro-acoustic anomaly" was detected in the ocean almost three hours after the last communication with the vessel on November 15, 30 miles (48 kilometers) north of its last known position.

Asked if the noise could have been an explosion, the spokesman declined to speculate, saying only: "It has to be corroborated and looked into."