Relatives of victims cry at the site of an explosion at a maternity hospital in Mexico City, on Jan 29. The explosion rocked the 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. [Photo /  Agencies]

MEXICO CITY - Seven were killed and at least 54 people were injured Thursday morning in a powerful gas tank truck explosion outside a maternity and children's hospital in Mexico City, local government said.

Four among the seven killed are babies and three are adults, the authority said.

Miguel Anger Mancera, mayor of Mexico City, told Televisa network that at least 54 people were injured, including 22 children.

The gas tank truck explosion happened at 7:15 a.m. local time (1315 GMT) outside the hospital and shattered 40 percent of the buildings in it.

Thirty-seven injured ones have been transported to other hospitals and the searching for victims buried under debris is going on.

The hospital built in 1993 is located in Zone Cuajimalpa, a densely populated area in the southwest of Mexico City, and usually have 100 people including the patients and staff.

The gas tank truck belongs to a company named Gas Expres Nieto. The explosion is caused by gas leak when the truck is offering service to the hospital, local authority said.

Mexican President Enrique Pena Nieto expressed his condolences to the victims and their families through this official Twitter account.