An Indian navy submarine with about 18 sailors on board has caught fire after an explosion and sank at a port in Mumbai.

Navy spokesperson Narendra Vispute said on Wednesday that efforts were being made to ascertain the safety of the personnel feared to be trapped on board the diesel-powered submarine.

"There are no reports of casualties at the moment," he added.

The cause of the explosion is being investigated, the navy spokesman said. Further details were not available.

Vinod Kumar Menon, city editor of Mid-Day newspaper based in Mumbai, said that the fire has been put off and cooling operation has begun.

Fire started at midnight and went on till 5:30-6:00am local time, he said.

Indian news channel, NDTV, said nearly a dozen fire engines rushed to the dockyard to douse the fire.

The submarine was nearly submerged at its berth in the naval dockyard.

Indian media reports say the 16-year-old Russian-made submarine, INS Sindhurakshak, had recently returned from Russia after undergoing an overhaul and upgrade.

Al Jazeera's Anu Anand, reporting from Rajouri, Kashmir, said that the cause of the explosion is still unknown.

Last year, India acquired a Russian Nerpa nuclear submarine for its navy on a 10-year lease from Russia at a total cost of nearly $1bn.

On Saturday, India activated the atomic reactor on its first indigenously designed and built nuclear submarine, which the navy could deploy in the next two years.

India has steadily built up its naval capabilities in recent years, spurred by its rivalry with neighbouring China.

Source: Al Jazeera and agencies