The fire broke out on the fourth floor of the Employees State Insurance Corporation Hospital in the suburb of Marol, filling the five-storey structure with smoke.

"Six people have died," a police official said.

"Almost all patients have been rescued. Operations are still on and the fire brigade is checking whether anyone is still trapped inside," the official added.

Mumbai Mayor Vishwanath Mahadeshwar told domestic news agency ANI, a Reuters partner, that the cause of the fire was not immediately known and would be investigated.

One fireman was injured during operations and patients were being moved to a nearby hospital, the police official added.