It was not known what caused the pre-dawn blaze at the sheet metal-and-cardboard shack in the densely populated neighborhood of Iztapalapa, on the sprawling capital's east side.

The fire was similar to another less than a month ago that was caused by a gas leak, said the top city official for the area, Clara Brugada.

"We're going to investigate (what caused the fire), but we will have to launch a campaign on how to use gas heaters safely in the winter," she said.

Authorities initially said one of the victims was an adult, but later clarified that all seven were minors.

Iztapalapa is the most populous area in Mexico City, with 1.8 million inhabitants, many of them living in destitute conditions. The capital's metropolitan area is home to more than 20 million people.