A gas explosion has ripped through a children's hospital in Mexico City, killing at least seven and injuring 54 more, including 22 children, officials say.

Firefighters toiled in the rubble of the maternity and children's hospital, which partly collapsed after a gas tanker truck blew up outside as it was making a delivery on Thursday, officials said.

"We have to conduct an evacuation for safety because a lot of the people are babies in the crib area," Mayor Miguel Angel Mancera told MVS Radio.

Fausto Lugo, the capital's civil protection chief, said two people died and that it was "very probable" that more were trapped in the rubble.

Mancera said one of the dead was a woman found under the collapsed structure.

He said the injured were not in serious condition and were being transferred to other hospitals.

Television images showed people with bloodied faces being carried to ambulances. Around 40 per cent of the hospital was destroyed, Mancera said.