UPDATED at 4:50 p.m. EST - A gas truck explosion decimated large parts of a maternity hospital on the western edge of Mexico City on Thursday, killing one woman and a child and leaving dozens injured, Mexico City Mayor Miguel Angel Mancera said.
The head of national emergency services earlier said that seven people had died, but the death toll was revised down by Mancera, who said there had been confusion around the number.
One of the people who was believed to have died was actually in very serious condition, he added.
The explosion destroyed around 70 percent of the hospital and injured 66 people, 22 of them seriously, Mancera said. Three people have been detained for their roles in the gas truck explosion, and two of them have been hospitalized, he added.
Several babies were found alive under the rubble but dozens of people were unaccounted for, as scores of rescue workers continued to scour through the concrete and twisted metal in search of survivors.
People seeking information on family members gathered around police lines that were set up to keep bystanders away from the chaotic scene.
"I am so worried about my sister, she's supposed to have given birth, we brought her in yesterday," said Monserrat Garduno, a 32-year-old nurse. "They won't let us pass, I want to know how she is."
Mexico City Mayor Miguel Angel Mancera said dozens of women and children were evacuated from the site, some with cuts from broken glass.
More ambulances waited at the scene to treat survivors pulled from the wreckage. Around 100 people were in the hospital at the time of the explosion, according to a city official. About 60 have been accounted for.
Ana Maria Sanchez, 47, wept as she stood in the crowd beyond the police lines, waiting to find out about her sister who worked in the hospital's administrative area. She said her calls to her sister's phone went straight to voice mail.
"I haven't heard anything about her, they have us just standing here," she said.
Some of the injured were evacuated by helicopter, and aerial footage showed firefighters scrambling over the skeletal wreckage of the building.
A leak in a hose from the truck, which was fueling the hospital's tanks, was believed to have triggered the explosion, officials said.
"They tried to stop the leak, but it was not possible," Mancera said.
President Enrique Pena Nieto voiced sadness and solidarity with the families of the victims on Twitter.
Many areas of Mexico City have no mains gas supply, and rely on deliveries from gas trucks.