A gas tank lorry has exploded outside a maternity and children’s hospital in Mexico City, killing a woman and a baby, the city’s mayor said.
The explosion, on the western edge of Mexico City, was so large that much of the building was destroyed.
Officials initially said seven people had been killed, but Mayor Miguel Angel Mancera later revised the figure.
Dozens of people were injured by flying glass. About 100 people were believed to be inside the hospital at the time.
Rescuers are at the scene, searching for anyone who might be trapped under rubble and twisted metal.
Mr Mancera said the blast had been caused by a leak in the hose carrying gas from the truck to the hospital kitchen.
The tanker workers had been struggling to repair the hose for about 15 to 20 minutes, according to reports.
As they worked, a large cloud of gas began to form outside the hospital, reports said.
Katy Watson, BBC Mexico and Central America correspondent reported earlier that following the deadly explosion, police blocked the road to the hospital and did not allow anyone through unless they were part of the medical or rescue services.
Federal police and firefighters went in. Some were armed with pickaxes and shovels, others with stretchers.
As conflicting reports emerged about the death toll, the rescue teams tried to sift through the rubble and make sure they had found all the survivors.
People I spoke to from the community said they were shocked. They also made a point of saying that the hospital was a public one, and that the patients who used it were those who did not have the money to pay for private healthcare.
Eyewitness Agustin Herrera, a 66-year-old anaesthesiologist, told the Associated Press news agency that the workers shouted for firefighters, but then there was a massive explosion.
At least 54 people were injured, 22 of them children.
Another eyewitness, Ismael Garcia, told AP there had been a “super explosion”.
He added: “Fortunately, we were able to get eight babies out.”
There have been a number of other fatal gas explosions in Mexico over recent years.
In February 2013, 37 people died in a blast at the headquarters of the state energy firm Pemex.