Divers were desperately trying to pry open the hatches of an Indian submarine in which several sailors died or were trapped after an explosion and fire on Wednesday in the navy's worst loss since the 1971 war with Pakistan.

Eighteen sailors were aboard the Russian-built INS Sindhurakshak when the blast struck after midnight.

The accident soured a week of naval milestones, including the launch of a locally built aircraft carrier aimed at giving the navy heft as it competes with China in the Indian Ocean.

Defence Minister A.K. Antony said crew members inside the diesel-electric Kilo-class submarine berthed at the main base in Mumbai had died.

But he gave no details, more than 12 hours after the incident, which revived memories of explosions on the Russian nuclear-attack submarine Kursk in the Barents Sea in 2000. Its entire crew of 118 died.

"There was an explosion immediately after midnight in the forward side of the submarine, where missiles and torpedoes are kept to be used whenever necessary," a naval source said.

"Now the submarine is tilted on the front side and all hatches are closed. They are trying to open those now."

He said one or two men are usually on duty on top of the submarine and they either jumped into the water or were thrown off by the force of the explosion. The number of crew in the boat when fully operational is 110.

Navy spokesman P.V.S. Satish earlier said efforts were under way to rescue trapped crew members. "We will not give up until we get to them," he said.

Photographs distributed by social media users appeared to show a large fireball over the navy dock.

The INS Sindhurakshak, which returned from an upgrade in Russia earlier this year, had suffered a similar accident in 2010 in which one sailor was killed while it was docked in the southern port of Visakhapatnam.

Typically, such a submarine is fitted with torpedoes and missiles. Torpedoes are launched underwater to attack other submarines while missiles are used for long ranges above water. There was no immediate word on the status of the weapons on board the Sindhurakshak.

"Lot of things are in very close proximity, there is fuel, there is hydrogen, there is oxygen, there are weapons with high explosives on board," said retired Indian navy chief Arun Prakash.

"So a slightest mistake or slightest accident can trigger off a huge accident. The question of sabotage - I mean, all possibilities have to be considered - but sabotage is probably the last possibility."

Another submarine in the Mumbai dock where these vessels are usually tied to each other suffered minor damage, the naval source said.

The last big loss for the navy was the sinking of the INS Khukri by a Pakistani navy torpedo during the 1971 war.

AGEING FLEET

India's navy has had far fewer accidents than the air force, dogged for years by crashes of Russian-made MiG-21 fighters.

However, most of the country's fleet of 15 submarines is in urgent need of modernisation and has been hampered by delays in government decisions as it battles corruption allegations.

Efforts to build a domestic arms industry to supply the military have made slow progress, with the country still the world's largest importer.

Earlier this week, a aircraft carrier slipped into the sea, though it is due to be fully operational only by 2017. The navy also announced that the reactor on its first indigenous nuclear submarine was now operational as part of the plan to build a powerful navy to counter China's growing presence in the Indian Ocean.

INS Sindhurakshak completed a 2-1/2 year upgrade at a Russian shipyard a few months ago. "This is a very, very old boat that really doesn't go out on long sea patrols," said Bharat Karnad, a senior fellow of national security studies, at the Centre for Policy Research.

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