Rescuers are racing against the clock to reach survivors trapped under the rubble of a school in Mexico City which collapsed during Tuesday’s earthquake.
One is a 12-year-old girl who is believed to be sheltering under a table, but rescuers say they do not know how to reach her.
At least 21 children and five adults died as the school collapsed.
The school was one of dozens of buildings toppled by the quake. So far 230 people are known to have died.
President Enrique Peña Nieto has declared three days of mourning for the victims.
As rescue operations continued for a second night, attention was focused on the Enrique Rébsamen primary school, in Mexico City’s southern Coapa district.
With anxious parents gathered outside awaiting news of missing children, civil protection volunteer Enrique Gardia announced that a thermal scanner had detected survivors trapped between slabs of concrete.
“They are alive! Alive!” he shouted.
“Someone hit a wall several times in one place, and in another there was a response to light signals with a lamp,” he added.
One mother, standing nearby waiting for news of her seven-year-old daughter, told reporters: “No-one can possibly imagine the pain I’m in right now.”
Later Jose Luis Vergara, who is co-ordinating the operation in the school, told local TV: “We know that there is a little girl alive inside but we don’t know how to reach her… without provoking a collapse or endangering personnel.”
Rescuers said they were able to speak to the girl, who said she was “very tired”, adding that there were other children nearby but she could not tell whether they were alive. She was also given water and oxygen.
An unnamed rescuer told AFP news agency he had seen five children alive but that they were trapped by metallic rods and the job of cutting them without hurting the children was “very delicate”.