Making a statement in the Rajya Sabha, the minister said that the cause of ignition is yet to be established.
"Initial assessments indicate that an internal explosion occurred in the forward compartment of the submarine where ordnance was stowed, resulting in further near-simultaneous explosions and major fire on board INS Sindhurakshak," he said.
Antony said that explosion and resultant damage were near instantaneous and 18 naval personnel, including three officers, and 15 sailors inside the submarine "could not evacuate".
He said the submarine was engulfed by a fire on the intervening night of Aug 13-14. Antony visited Mumbai Aug 14 to obtain a first-hand account of the situation.
"Preliminary investigations indicate that the explosion was due to the possible ignition of armament. The cause of ignition is, however, yet to be established," Antony said.
He said visual and forensic examination would throw more light on the possible cause of ignition.
"This will be possible only after the submarine is afloat and dewatered. A board of inquiry with all relevant specialists has been constituted to investigate the likely causes of accident at the earliest," he said.
Antony also made a statement on the "unprovoked attack" near the Line of Control (LoC) in Jammu and Kashmir Aug 6 that resulted in the killing of five Indian soldiers.
The minister had made a similar statement on the LoC incident in the Lok Sabha on Aug 8.
Antony said specialist troops of the Pakistan army were involved in the attack.