Story highlights The defense minister confirms some deaths but doesn't say how many

About 18 people were reportedly on board when the blast happened

Rescue efforts are still under way

The Russian-built submarine has mostly sunk following the explosion and blaze

An unspecified number of Indian sailors have died after an explosion and fire on a navy submarine at a dockyard in Mumbai, authorities said Wednesday.

Rescue efforts are under way at the scene following the blast and "major fire" early Wednesday on the INS Sindhurakshak, the defense ministry said in a statement.

Defense Minister A.K. Antony later said that some officers had lost their lives, but he didn't give any details on how many had died.

The Russian-built submarine has mostly sunk from the damage, the defense ministry said, and only a part of it is visible. About 18 people were on board the vessel at the time of the blast, it said.

Images carried by CNN sister network IBN showed the night sky over the port area lighted up by the explosion and blaze, which began shortly after midnight.

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Navy and civilian firefighters were called into action at Mumbai's naval dockyard to tackle the flames and rescue survivors. By Wednesday afternoon, the navy had taken sole responsibility for the rescue effort, Mumbai fire chief Suhas Joshi said.

An inquiry has been ordered to determine the cause of the explosion, the defense ministry said.

Refitted in Russia

The Sindhurakshak was commissioned in 1997, according to the Indian Navy.

It was refitted in a Russian shipyard between August 2010 and January of this year, Russian state new agency Itar Tass reported.

The redesign included the installation of new missile, communication and cooling systems, as well as a new radar, the agency said.

It reported that the upgrade also included other measures to improve the submarine's combat quality and operational safety.

The crisis at the Mumbai dockyard Wednesday comes after India launched a home-built aircraft carrier Monday in a bid to join a select group of nations capable of building such warships.