Mexico's navy says there are no missing children at a collapsed Mexico City school where rescuers have been hunting for a girl they believed to be trapped.

Assistant Navy Secretary Angel Enrique Sarmiento says there is evidence of a person who may still alive, but he says it's probably a school worker.

The search for the supposedly missing girl has been a focus of attention across the country as a symbol of hope following Tuesday's magnitude 7.1 earthquake.

Mr Sarmiento says 11 children were rescued alive after the quake, while 19 children and six adults died.

Rescuers are reinforcing the shaky wreckage of the school, which is in the city's south.

The remains of the building had shifted dangerously earlier Thursday morning, prompting some rescuers to evacuate the top of the pile.

At least 245 people were killed in Tuesday's earthquake, according to the most recent official count.