The Russian firm that refitted an Indian submarine which exploded and sank in a Mumbai dock with 18 sailors on board said Wednesday the craft had been fully operational when returned to India in January.

>>Indian submarine explodes with 18 on board

A spokesman for the Russian Zvyozdochka ship repair company told RIA Novosti that "certain issues" had been raised when the Russian-made INS Sindhurakshak was inspected by experts at the Severodvinsk port on the Barents Sea.

The unnamed spokesman did not give further details about the nature of the issues but said they were part of the "normal working process".

He said India raised no objections about the state of the diesel-powered vessel when receiving it from Russia.

"We signed a contract for a light overhaul and modernisation in June 2010, and completed (the refit) in January 2013," the Zvyozdochka spokesman told RIA Novosti.

"During the repairs, we fitted a new Club rocket complex and a number of foreign systems on the ship, including Indian ones."

The spokesman said these included updates to the submarine's navigation and communication systems as well as an overhaul of its power generator.

Officials said the submarine had undergone succesful post-repair tests -- including on its weapons systems -- in the Russian sector of the White Sea in November and December.

The INS Sindhurakshak is still covered by a Russian warranty and eight Zvyozdochka employees were currently in the Mumbai port where the vessel sank early Wednesday.

"We still have no information about what really happened there," the Zvyozdochka spokesman said.

Another Zvyozdochka official said experts at the firm would conduct an emergency meeting over the accident later on Wednesday.

"Zvyozdochka is prepared to render its full assistance in the investigation and search for the causes of the accident," a company spokesman told ITAR-TASS.

The INS Sindhurakshak is a Kilo-class submarine under NATO classifications and was contracted by India in 1995.

Construction on the boat was completed at the Saint Petersburg's Admiralty Wharf in 1997.

Russian news reports said that the boat began its return voyage to India on January 29 under its own power after undergoing the refit.

The boat docked in Mumbai on April 29.

ITAR-TASS reported at the time that the updates had "improved the submarine's offensive qualities and operational safety."

The Zvyozdochka repair company said it had successfully refitted five Russian-made diesel-powered Indian submarines since 1997.