Mumbai, Aug 14 (IANS): An Indian Navy submarine, which underwent a major refit in Russia, was rocked by a massive explosion and engulfed in flames Wednesday at a high security dockyard in Mumbai. Many sailors jumped off to safety, but 18 were feared to be trapped inside.

The deafening blast followed by a fire occurred shortly after midnight on the INS Sindhurakshak submarine at the naval dockyard, officials said.

Many sailors on board the submarine reportedly jumped off to safety, but sources added 18 sailors were feared to be trapped in the vessel.

"...The details are being ascertained," an official defence ministry statement issued at 3.15 a.m. said.

Some INS Sindhurakshak sailors have been rushed to naval hospital INHS Ashvini in Colaba.

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 comes within days of India getting its first indigenous
aircraft carrier Vikrant Aug 12 and the nuclear reactor that propels Arihant, the country's first ballistic missile submarine, going critical Aug 9.

The cause of the explosion and blaze, as well as the extent of damage to the submarine and naval properties, is still not known.

A board of inquiry has been ordered.

TV grabs showed a huge ball of flame.

At least 16 fire tenders of Mumbai Fire Brigade and Mumbai Port Trust were rushed to assist the naval fire brigade to douse the conflagration, the smoke emanating from which could be seen in many parts of south Mumbai.

A fire brigade official said that around 3 a.m., the blaze had been brought under control and prevented from spreading to other vessels docked in the vicinity.

The blast was first heard ashore by deputy chief fire office P.S. Rahandale who was on leave and attending a private engagement near the Gateway of India promenade. He immediately alerted the fire brigade and other emergency services.

Navy chief Admiral D.K. Joshi "will be visiting the dockyard", sources said.

The contract for the refit and modernisation of the INS Sindhurakshak (S63), a Project 877 EKM (NATO Kilo-class) submarine was signed in June 2010, RIA Novosti news agency had earlier reported.

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.

In addition, 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".

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 boat displaces 2,300 tonnes, carries 52 crew members, has a top speed of 19 knots (35 km per hour) and diving depth of 300 metres.

Zvezdochka, which specialises in repair and refit of nuclear-powered boats, has already refitted four of India's diesel-electric fleet -- Sindhuvir (S58), Sinduratna (S59), Sindhugosh (S55) and Sindhudvhaj (S56).