About 200 people lived in the poorly built 15-year-old structure in the city's Lalita Park area. Most of them were migrant laborers, some of whom were sleeping inside when the building collapsed around them Monday night. About 30 people are still trapped alive, their cries audible from deep under piles of crumbled cement and bricks, The New York Times reported. Some 80 people have been injured.
"They took more than 45 minutes to reach the site. And then there was confusion about how they were going to bring in the ambulances," resident Mohinder Singh told The Associated Press.
An adjacent building that's also in danger of collapse has been evacuated, police official Mohammed Akhlaq told the AP.
The cause of the collapse is under investigation, but city officials told CNN the building's foundation was weakened by recent monsoon rains. The apartment complex sits close to the Yamuna River, which swells during India's rainy season.
"I believe it's an unauthorized building," Sheila Dikshit, the chief minister of New Delhi, told CNN. "I believe there was water in the bottom of the basement for many days."
"The scale of the tragedy is unprecedented. I don't think such a tragedy has taken place in Delhi in the recent past," Dikshit told the Times.
Charges have been filed against the building's owner, and police are searching for him, the AP said. Illegal building for residential use is common in New Delhi, a city of 17 million people that suffers from corruption and weak enforcement of local zoning laws.