An Indian naval submarine armed with missiles and torpedoes exploded in Mumbai port early yesterday and sank, sending fireballs into the night sky over the country’s commercial capital and killing up to 18 sailors trapped on board.

The navy immediately ordered an inquiry into the disaster on the INS Sindhurakshak, one of a fleet of diesel-powered Kilo-class submarines bought from Russia. A second submarine nearby caught alight, according to a fire officer, although the extent of the damage was not immediately known.

If the blast was an accident, as suggested by the navy, the explosion would revive concerns about the reliability and safety of India’s military equipment and its maintenance procedures as it seeks to modernise its armed forces and compete with China in the Asia-Pacific region.