NEW DELHI — An explosion ripped through a diesel-powered Indian submarine Wednesday, killing at least some of the 18 sailors on board and leaving the vessel partly submerged at dock in Mumbai.

Speaking outside parliament as he prepared to leave for the accident site, Defense Minister A.K. Antony said he felt “very sad” about the deaths of some sailors, without specifying the number killed. The explosion and subsequent fire occurred inside the INS Sindhurakshak just after midnight, the navy said in an e-mailed statement.

The blast is a setback for the Indian navy as the country seeks to bolster its military amid a buildup by neighbor China. On Aug. 10, India activated the atomic reactor on its first indigenously built submarine, and two days later unveiled its first homegrown aircraft carrier.

“The navy has certainly lost some of the sheen it has built up over the last week,” said Uday Bhaskar, a former Indian Navy commodore and now a defense analyst at the Society for Policy Studies in New Delhi. “Compared with other navies that operate submarines, India has a very good track record.”

Bhaskar said the explosion was the worst-ever disaster for India’s submarine program. The Sindhurakshak, whose name means “protector of the ocean,” is a Russian-built Kilo class submarine that returned to Mumbai earlier this year after a $133 million refit at a shipyard in Russia, according to India’s defense ministry.

In comments made to the NDTV 24x7 television network, former Navy Chief Sushil Kumar said the vessel was one of India’s frontline subs. The blast occurred at the Navy Dockyard in Mumbai less than a mile from the financial capital’s luxury Taj Mahal hotel.

BATTERY FIRE

“Rescue operations are on. We have not yet reached inside the submarine,” said P.V.S. Satish , a spokesman for the Indian Navy.

In 2010, a sailor on board the submarine was killed by a fire that broke out in the battery compartment while it was docked at naval base in southern India, Press Trust of India reported at the time. The ship was sent for an overhaul in Russia later that year.

India plans to increase its defense spending by 14 percent in the next financial year as it modernizes its military to counter traditional rivals as well as China’s rising power.

In the last week, India has achieved two military milestones, activating the nuclear reactor aboard the INS Arihant submarine, and on Aug. 12 unveiling its first home-built aircraft carrier. The 37,500-ton vessel was unveiled at a shipyard in the southern city of Kochi but won’t enter active service for several years.

INDIAN FLEET

India’s navy has a fleet of 15 submarines, including 10 diesel-electric vessels similar to Sindhurakshak. They have a maximum diving depth of 300 meters (984 feet), a top speed of 18 knots and are able to operate for 45 days with a crew of 53 people, according to the navy’s website.

India last year inducted a Russian-built nuclear-powered attack submarine into its navy at a cost of $900 million that allows longer missions and a faster response to threats. The rest of the submarines are more than a decade old, with half of the fleet commissioned in the 1980s.