India’s navy chief held out little hope for survivors on a submarine after some of its weapons detonated accidentally and fire swept through it.

The likely deaths and damage are the worst blow to the navy since a 1971 war with Pakistan.

Eighteen sailors were on board the 16-year-old, Russian-built INS Sindhurakshak, which was docked at the main naval base in Mumbai when two blasts rocked the vessel in the middle of Tuesday night.

The accident spoiled a week of modernisation triumphs for the navy, including the launch of a locally built aircraft carrier aimed at giving India the edge at sea as it competes with China in the Indian Ocean.

Navy chief Admiral D.K. Joshi said divers had managed to pry open the main hatch of the diesel-powered sub, more than 12 hours after the incident, and were trying to find their way through the vessel.

“Whilst we hope for the best, we are prepared for the worst... there is a possibility, however remote it could be, of an air pocket. There is a possibility, however remote it might be, of someone having grabbed a breathing set,” he told a news conference.

The INS Sindhurakshak, which returned from an upgrade in Russia this year, had suffered an accidentin 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.

Whilst we hope for the best, we are prepared for the worst

“Just short of midnight, there were two rapid and near-simultaneous major explosions on board the submarine, which resulted in a major and rapid spread of fire on board,” Joshi said. “It is some of the ordinance on board that seem to have exploded.”

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