A gas truck explosion decimated large parts of a maternity hospital west of Mexico City on Thursday, killed at least seven people, including three children, and leaving many others missing, officials said.

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.

Members of the public who sought information on family members gathered around the 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.

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.