MUMBAI (AFP) - A diesel-powered Indian submarine exploded and sank Wednesday in a Mumbai dock, killing an unknown number of 18 crewmen on board and setting back the navy's ambitious modernisation drive.
The fully-armed INS Sindhurakshak, returned by Russia earlier this year after a major refit, is nose-down in the water, with just a small part visible above the surface, the navy said.
The disaster is thought to be the worst for the Indian navy since the sinking of a frigate by a Pakistani submarine in 1971 and India's defence minister described the explosion as the "greatest tragedy in recent time".
"I feel sad about those navy personnel who have lost their lives in service of the country," Defence Minister A.K. Antony told reporters in New Delhi without saying how many had died.
The blast came days after New Delhi trumpeted the launch of its first domestically-produced aircraft carrier and the start of sea trials for its first Indian-made nuclear submarine.
The world's biggest democracy has been expanding its armed forces rapidly to upgrade its mostly Soviet-era weaponry and react to what is perceived as a growing threat from regional rival China.
Grainy amateur video footage taken by a witness showed a fireball in the forward section of the Russian-made INS Sindhurakshak where torpedoes and missiles are stored as well as the battery units.
"There were two to three explosions and the night sky lit up briefly," said eyewitness Dharmendra Jaiswal, who works in a public toilet near the dockyard and was sleeping there overnight.
"There was a lot of smoke and I thought it was some major repair work," he told AFP near the scene of the disaster on the southern tip of the Mumbai peninsula.
P.S. Rahangdale, an off-duty firefighter who rushed to the scene, told a local television channel that the INS Sindhurakshak "was totally on fire" and was berthed next to another submarine.
"Because of timely intervention of my team and resources and navy's resources we could save that second submarine," he said.
The navy stressed that the cause of the explosion was not known and divers were working to enter the stricken hull of the vessel, which is resting on its nose on the seabed eight metres (26 feet) down.
Other sailors stationed on vessels berthed nearby were admitted to a navy hospital with burn injuries.
"Eighteen sailors were on board the submarine, they have not been evacuated yet," navy spokesman P.V. Satish told AFP.
The submarine was fully operational and was therefore carrying a "full complement of torpedoes and missiles", he said.
In February 2010, the INS Sindhurakshak also suffered a fire while docked in Visakhapatnam city in southern India, killing a 24-year-old sailor and leaving two others with burns.
An inquiry into the cause of the explosion has been ordered and Defence Minister Antony and India's navy chief arrived at the scene in the afternoon.
One senior naval officer, speaking on condition of anonymity, said the "needle of suspicion" was on the battery system as the source of the fire, but he stressed that these were early assumptions.
A spokesman for the Russian Zvyozdochka company which overhauled INS Sindhurakshak's weapons, navigation and power generator systems said that India raised no objections about the vessel when it was returned after testing in April.
The submarine, whose name means "Protector of the Seas" in Hindi, is still covered by a Russian warranty and eight Zvyozdochka employees are still in Mumbai, the spokesman said.
"Zvyozdochka is prepared to render its full assistance in the investigation and search for the causes of the accident," the spokesman told the ITAR-TASS news agency.
The submarine was built in 1997 at the Saint Petersburg's Admiralty Wharf.
Arun Prakash, a former Indian navy chief, said the prospect of survivors appeared unlikely.
"There is a possibility that these 18 crewmen may have sealed themselves off in some part of the submarine and they may still have survived," he told the CNN-IBN news channel.
"Otherwise with this massive explosion chances don't look very bright," he added.
Rahul Bedi, a defence expert with IHS Jane's Defence Weekly, told AFP the 16-year-old submarine lacked some modern safety features common to newer vessels even after its $80 million overhaul.
"They don't have escape routes in the event of accidents unlike some of the modern submarines," he said.
The Indian navy says it has a total of 14 submarines but only between seven and nine are operational at any point because of regular repair and refitting operations.
The disaster had echoes of a tragedy in Russia in 2000 when the Kursk nuclear submarine sank in the Barents Sea with the loss of all 118 crew on board.
Russia is still the biggest military supplier to India, but relations have been strained recently by major delays and cost over-runs with a refurbished aircraft carrier, the INS Vikramaditya.The accident is set to overshadow scheduled talks between Indian Prime Minister Manmohan Singh and Russian President Vladimir Putin at a G20 summit in St. Petersburg in September.