At least 18 sailors remained trapped after a Russian-made Indian Navy submarine caught fire, exploded and sank at midnight Wednesday in a major marine base in downtown Mumbai, which was described by local media as the worst navy disaster since 1971 and a major setback for the Indian Navy.

The INS Sindhurakshak just underwent a major refit in Russia and sailed back to India via the Suez Canal, where it got unspecified "help" from Egypt, said local TV channel Times Now.

It was rocked by a massive explosion said to be caused by its battery problem and sank off the Lion Gate Dockyard here. While some jumped off to safety, 18 were trapped inside the sunk vessel, navy official sources told local media.

Some injured sailors have been rushed to naval hospital INHS Ashvini in Colaba, south Mumbai.

INS Sindhurakshak is an Indian Navy diesel-electric submarine that has been handed back to India following a major refit at Russia's Zvezdochka shipyard.

The explosion on the submarine came one day after India launched the hull of its first indigenous aircraft carrier Vikrant and four days after it activated the nuclear reactor that propels Arihant, the country's first ballistic missile submarine.

The cause of the explosion and blaze, as well as the extent of damage to the submarine and naval properties, is yet to be determined, said navy officials.

Meanwhile, the Indian Navy has set up a board of inquiry into the incident.

TV footages showed a huge ball of flame lighted the dock at the time of the fire while fire engines were heard throughout the night. Smoke emanating from the dock invaded many parts of the high class and tourists drawing Colaba.

This is the first time an Indian navy vessel sank since the 1971 Bangladesh War.

According to Indo-Asian News Service, part of the refit involved installation of equipment for Klub-S (3M54E1 anti-ship and 3M14E land attack) cruise missiles and over 10 Indian and foreign-made systems, including the Ushus hydro-acoustic (sonar) system and CSS-MK-2 radio communications system.

The boat's cooling system was modified, a "Porpoise" radio- locater fitted and other work carried out "increasing the boat's military capacity and safety", said the news service.

The Sindhurakshak was laid down in one of Russia's oldest shipyards, the Admiralty Wherf yard in St. Petersburg in 1995. It was launched in 1997 and delivered in December that year.

The ship displaces 2,300 tons, carries 52 crew members, has a top speed of 19 knots (35 km per hour) and diving depth of 300 meters.