Malfunctioning and ageing batteries were the likely cause of fire on an Indian submarine that left two dead this week, reports said Friday, amid warnings that the navy is "on the verge of breakdown".

Wednesday's accident on board the Russian-built INS Sindhuratna, the latest in a string of naval mishaps, left seven crew members injured, while two bodies were found after the stricken submarine docked in Mumbai on Thursday.

While the vessel had undergone a refit earlier this year, the batteries that powered it underwater had not been replaced because of contracting delays, The Hindu daily said, citing "highly placed navy sources".

The newspaper said hydrogen leaking from the batteries was thought to have caused the explosion.

Admiral D.K. Joshi, the chief of naval staff, announced Wednesday night he was standing down to take "moral responsibility" for the recent run of recent naval accidents.

The most deadly of these happened last August, when 18 sailors were killed as the fully-armed submarine INS Sindhurakshak exploded in flames and sank in a military shipyard in Mumbai.

"India's military is, literally, on the verge of breakdown," said an editorial in The Hindu, adding that "acquisitions of desperately needed armour and artillery systems have been endlessly delayed".

The Times of India, which also reported a likely battery malfunction, said in an editorial that "poor maintenance of submarines and warships is crippling India's navy".

It said 12 mishaps involving submarines and warships over the last seven months "may also severely impair India's ambitions of becoming a strategic blue-water power able to operate far beyond its extensive coastline".

Various other naval accidents have been reported in recent months including a submarine running aground in Mumbai's harbour, fires on a minesweeping vessel and an aircraft carrier, and a collision between a frigate and a fishing boat.

INS Sindhuratna is a kilo-class submarine which normally operates with a crew of 53 and can sail on its own for 45 days, the navy's website says.

It had been undergoing trials off the Mumbai coast as part of a clearance process for full operations when the incident occurred.