Rescuers hold a firefighteras he cuts away debris from a flattened building during a search for survivors on Tuesday after a powerful earthquake hit south-central Mexico. YURI CORTEZ / AFP

MEXICO CITY - Desperate rescue workers scrabbled through rubble on Wednesday in a floodlit search for dozens of children feared buried under the rubble of a Mexico City school, one of hundreds of buildings wrecked by the country's most lethal earthquake in a generation.

The magnitude-7.1 quake killed at least 225 people, nearly half of them in the capital, 32 years to the day after a devastating 1985 earthquake and less than two weeks after a powerful tremor killed nearly 100 people in Mexico's south.

China has extended deep condolences to the victims and expressed heartfelt sympathy to their bereaved families, the Mexican government and people in affected areas, Foreign Ministry spokesman Lu Kang said on Wednesday. "China has paid attention to the earthquake and the heavy casualties it has caused," Lu said, adding that "China believes that the Mexican people can overcome the disaster and rebuild their lives".

Lu said that some Taiwan compatriots have been trapped in Mexico and one was injured. "The Chinese Foreign Ministry and local Chinese embassy will continue to pay attention to the disaster and offer help to Chinese citizens."

Amid the twisted concrete and steel ruin of the Enrique Rebsamen school, soldiers and firefighters found 22 children and two adults dead, while another 30 children and 12 adults remained missing, President Enrique Pena Nieto said. Bulldozers moved rubble under the buzz and glare of floodlights powered by generators, as parents of students at the school clung to hope their children had survived.

"They keep pulling kids out, but we know nothing of my daughter," said 32-year-old Adriana D'Fargo, her eyes red after hours waiting for news of her 7-year-old.

Parts of colonial-era churches crumbled in the state of Puebla, where the US Geological Survey located the quake's epicenter, about 160 kilometers southwest of the capital, at a depth of 51 km.

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