MEXICO CITY (UPDATE) - Fifteen people were killed and about 70 injured when a railway overpass collapsed onto a busy road in Mexico City on Monday night.

Rescue efforts for potential survivors were paused shortly after the accident, authorities said, because of the risk that more of the metro overpass and train cars could slam down onto the road.

A video on local channel Milenio TV showed the structure plummeting onto a stream of cars near Olivos station in the southeast of the city at around 10.30 p.m. local time, sending up clouds of dust and rubble.

Other images showed at least two train carriages precariously hanging from the damaged overpass as emergency fire and medical crews initially used ladders to access the carriages.

However, Mexico City Mayor Claudia Sheinbaum said the rescue had been suspended "because the train is very weak", adding it appeared the accident was caused by a girder giving way.

A crane was being transported to the site to stabilise the train carriages so rescuers could resume their work, she said.

"So far we have 15 people who unfortunately lost their lives," Sheinbaum told reporters at the accident site, adding that authorities believe more are trapped.

Among the injured, 34 people were transported to hospital, she said.

The Metro 12 line that runs over the collapsed overpass was built almost a decade ago when Foreign Minister Marcelo Ebrard was mayor of Mexico City.

"What happened today with the Metro is a terrible tragedy. My solidarity is with the victims and their families," Ebrard said on Twitter. "Of course, the causes must be investigated and responsibilities defined."

Ebrard and Sheinbaum are seen by many political observers as the most likely successors to President Andres Manuel Lopez Obrador once his six-year term is over in 2024.

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