The deputy chief fire officer, Sunil Choudhary, said the number of injured stood at “more than 25 people”. Choudhary added that some had jumped from the burning building.
Broadcaster NDTV, meanwhile, reported that more than 40 people had sustained burns and were hospitalised.
The fire broke out in the first floor of the building that houses the office of a surveillance camera manufacturing company, police said.
Amit Mahajan, whose cousin and nephew were missing after the fire, said they had been at the building to “train workers”.
“I have visited all the nearby hospitals but there is no trace of them. We are not sure about their safety or whereabouts,” he said.
Fires are common in India due to poor building practices, overcrowding and a lack of adherence to safety regulations.
The Indian capital of 20 million people is currently suffering through a heatwave, with temperatures of up to 46C recorded in some places.
The prime minister, Narendra Modi, offered condolences on Twitter.
“Extremely saddened by the loss of lives due to a tragic fire in Delhi. My thoughts are with the bereaved families. I wish the injured a speedy recovery,” Modi said.
The Delhi chief minister, Arvind Kejriwal, tweeted: “Shocked and pained to know abt this tragic incident. I am constantly in touch [with] officers. Our brave firemen are trying their best to control the fire and save lives.”
With Agence France-Presse and Reuters