Explosion sinks Indian submarine INS Sindhurakshak (VIDEO)

An Indian naval submarine burst into flames and sunk in a Mumbai port early on Wednesday, with 18 crew members on board.

At least some of the sailors were killed, said India's defense minister, A.K. Antony, without giving further details, though as more time passes the sailors' chance of survival decrease. 

"Whilst we hope for the best, we are prepared for the worst," said Navy chief Admiral D.K. Josh. "There is a possibility, however remote it could be, of an air pocket. There is a possibility, however remote it might be, of someone having grabbed a breathing set."

The 16-year-old INS Sindhurakshak had recently returned to the Indian port after receiving an overhaul and an upgrade in Russia, where it was originally built, the Associated Press reported.

It is the second explosion aboard the submarine. In 2010, a hydrogen leak in a faulty valve caused an explosion in the battery compartment that killed one sailor and injured two others.  

According to the Times of India, Wednesday's incident began with an explosion shortly after midnight, which caused the submarine to catch fire and then become partially submerged, with many crew members still trapped inside. 

The cause of the explosion is not clear. A naval inquiry has been opened, the BBC said, though until water can be drained from the submarine, it will be difficult to understand exactly what happened. 

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Many of the sailors on the afflicted craft were able to jump to safety, the Times added, while the fates of the other crew members remain unknown. 

“However, there is likelihood of some personnel being trapped inside. The details are being ascertained,” a defense ministry statement issued at 3:15 a.m. Wednesday stated, according to The Hindu.

The smoke coming from the blast could be witnessed by civilians in South Mumbai, said the Times, while city fire engines rushed to the scene to assist the naval fire brigade with quenching the flames. 

Only days earlier, India announced it had activated the reactor aboard the INS Arihant, an Indian designed-and-built nuclear submarine, and the first such craft constructed by any country outside the major military powers of France, the US, the UK, Russia and China.

Prime Minister Manmohan Singh said then that the submarine represented a "giant stride in… our indigenous technological capabilities," according to NPR. 

The explosion happened one day ahead of celebrations for India's Independence Day, and officials are now on alert for possible terror attacks.

This is a turbulent birthday for India,” C. Uday Bhaskar, a retired Indian Navy commodore with the National Maritime Foundation, told The New York Times. “The security challenges for this country are complex and mounting.”