A man watches Indian Navy submarine INS Sindhurakshak on fire in Mumbai August 13, 2013. [Photo/Agencies]

MUMBAI - Eighteen Indian sailors were trapped after an explosion and fire on board a conventionally powered Indian submarine berthed at the coastal city of Mumbai early on Wednesday, the navy said.

"There are some people who are trapped on board, we are in the process of trying to rescue them," said navy spokesman P.V.S. Satish. "We will not give up until we get to them."

The explosion just after midnight inside the kilo-class INS Sindhurakshak was likely an accident, but an investigation was under way to establish the cause, Satish said.

The accident comes in the same week as India launched a locally built aircraft carrier due for completion in 2017, and announced that the reactor on India's first home-built nuclear submarine was now operational.

"The Sindhurakshak today is perhaps the worst accident that the Indian navy's submarine arm has ever had," said defence expert and former commodore Uday Bhaskar, adding that it "takes some of the sheen away" from the navy's latest achievements.  

India's navy has had far fewer accidents than the air force,

dogged for years by crashes of Russian-made MiG-21 fighters.

However, the country's fleet of 14 submarines is in urgent need of modernisation, and the INS Sindhurakshak had returned just a few months ago from a 2-1/2 year upgrade at a Russian shipyard. Satish said the submarine fleet had an "impeccable" safety track record, though media reports said there was an explosion on the same vessel in 2010 that killed a crew member.

Photos distributed by social media users appeared to show a large fireball over the navy dock where the diesel-electric submarine was berthed.

"There was a loud explosion post midnight and I woke up," said Dharmendra Jaiswal, who manages a public toilet opposite the naval dockyard. "I could see the skyline was bright and I could make out that some fire or blast had occurred inside."

The vessel, which was fully operational with weapons on board, was half-submerged after the fire. A team of navy divers was mobilised to search for survivors and 16 fire tenders were brought in to put out the blaze, local media said.

Three people who were near the submarine at the time of the explosion were injured and being treated in hospital, spokesman Satish said.