(Updates death toll, clarifies cause of explosion)

MEXICO CITY, Jan 29 (Reuters) - An explosion rocked a maternity hospital west of Mexico City Thursday when a leak from a gas truck ignited, destroying a large part of the building and killing at least seven people, including three children, officials said.

Several babies were found alive under the rubble, they said.

Luis Felipe Puente, head of the country's civil protection agency, confirmed the seven deaths. Rescue workers were scouring through concrete and twisted metal for survivors.

Mexico City Mayor Miguel Angel Mancera said dozens of women and children were evacuated from the site, some with cuts from glass.

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 that 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. (Reporting by Michael O'Boyle and Anahi Rama; Editing by Simon Gardner, Chizu Nomiyama and Bernadette Baum)