Mexican rescuers are working to reach a 12-year-old girl trapped beneath a collapsed school and to save other possible survivors buried in rubble following the country's most deadly earthquake in three decades.

At least 237 people were killed by the magnitude 7.1 earthquake, which struck about 90 miles (150km) southeast of Mexico City on Thursday afternoon, 32 years to the day after a 1985 quake killed thousands.

Rescue workers were able to communicate with the girl, identified as Frida Sofia, who said there were two other students nearby, but she could not tell if they were alive, according to broadcaster Televisa.

The sight of her wiggling fingers became a symbol for the hope driving thousands of professionals and volunteers to work frantically at dozens of wrecked buildings across the capital and nearby states looking for survivors.

Thermal imaging suggested several more people might be in the airspace around her.

Mexico earthquake: Many die in school collapse

A volunteer rescue worker, Hector Mendez, said cameras lowered into the rubble suggested there might be four people still inside, but he added that it wasn't clear if anyone beside the girl was alive.

The girl's full name was not made public, but her family waited in anguish nearby, knowing the bodies of 21 school children and four adults hd already been recovered from the Enrique Rebsamen School.

They and other parents clung to hope after rescue teams reported a teacher and two students had sent text messages from within the rubble.

Rescuers moved slowly, erecting makeshift wooden scaffolding to prevent rubble from crumbling further and seeking a path to the child through the unstable ruins. As in rescue scenes throughout the central Valley of Mexico, officials ruled out using heavy equipment as long as there were signs of life below.

They periodically demanded "total silence" bystanders, who would freeze in place and stay quiet, to better hear calls for help.

Mexico City earthquake in pictures Show all 13 1 /13 Mexico City earthquake in pictures Mexico City earthquake in pictures A handout picture provided by the citizen Edgar Cabalceta shows a general view of Mexico City following a magnitude 7.1 earthquake, in Mexico City, Mexico EPA Mexico City earthquake in pictures Rescuers and residents look for victims amid the ruins of a building knocked down by a magnitude 7.1 earthquake that jolted central Mexico damaging buildings, knocking out power and causing alarm throughout the capital on September 19, 2017 in Mexico City, Mexico. The earthquake comes 32 years after a magnitude-8.0 earthquake hit on September 19, 1985 Getty Images Mexico City earthquake in pictures A police officer stands guard near a building which collapsed after a quake rattled Mexico City RONALDO SCHEMIDT/AFP/Getty Images Mexico City earthquake in pictures Rescuers, firefighters, policemen, soldiers and volunteers remove rubble and debris from a flattened building in search of survivors after a powerful quake in Mexico City AFP/Getty Images Mexico City earthquake in pictures Rescuers and residents look for victims amid the ruins of a building knocked down by a magnitude 7.1 earthquake that jolted central Mexico damaging buildings, knocking out power and causing alarm throughout the capital on September 19, 2017 in Mexico City Getty Images Mexico City earthquake in pictures People try to rescue survivors from a collapsed buildings after a magnitude 7.1 earthquake on the Richter scale in Mexico City, Mexico, 19 September 2017. At least 79 people lost their lives in the states of Morelos, Puebla and Mexico by the earthquake of magnitude 7.1 on the Richter scale. EPA Mexico City earthquake in pictures People clear rubble after an earthquake hit Mexico City, Mexico REUTERS Mexico City earthquake in pictures People clear rubble after an earthquake hit Mexico City, Mexico REUTERS Mexico City earthquake in pictures Rescuers and residents assists an injured victim amid the ruins of a building knocked down by a magnitude 7.1 earthquake that jolted central Mexico damaging buildings, knocking out power and causing alarm throughout the capital on September 19, 2017 in Mexico City, Mexico. The earthquake comes 32 years after a magnitude-8.0 earthquake hit on September 19, 1985. Getty Images Mexico City earthquake in pictures People remove debris outside a collapsed building after an earthquake in Mexico City, Mexico REUTERS Mexico City earthquake in pictures People remove debris as they search for possible victims after a quake rattled Mexico City on September 19, 2017. A powerful earthquake shook Mexico City on Tuesday, causing panic among the megalopolis' 20 million inhabitants on the 32nd anniversary of a devastating 1985 quake. The US Geological Survey put the quake's magnitude at 7.1 while Mexico's Seismological Institute said it measured 6.8 on its scale. The institute said the quake's epicenter was seven kilometers west of Chiautla de Tapia, in the neighboring state of Puebla. AFP/Getty Images Mexico City earthquake in pictures A security guard walks over debris of a building which collapsed during a quake in Mexico City AFP/Getty Mexico City earthquake in pictures Reconstruction work in the towns affected by the telic movement has begun. Several temples suffered severe damage. Rex

Similar efforts have pulled more than 50 survivors from buildings around the country, President Enrique Pena Nieto said in a national address.

A man was pulled alive from a partly collapsed apartment building in northern Mexico City more than 24 hours after the quake and taken away in a stretcher, apparently conscious.

Fifty-two buildings collapsed in Mexico City alone, Mayor Miguel Angel Mancera told reporters.

Rescue workers search for children trapped inside the collapsed Enrique Rebsamen school in Mexico City (AP Photo/Carlos Cisneros)

The earthquake struck a mere 32 miles (31km) beneath the surface, sending major shockwaves through the metropolitan area of some 20 million people. Much of the capital is built upon an ancient lake bed that shakes like jelly during a quake.

Initial reports suggest that collapses were limited mostly to buildings that predated the 1985 quake, after which stricter building codes were enacted.

Mexico was still recovering from another powerful tremor that killed nearly 100 people in the south of the country less than two weeks ago.

Reinforcements from other countries began to arrive and more were on the way.

The United States, Israel, Spain, Japan and several Latin American countries had already responded with technical assistance and rescue teams, the president said.