Deadly aftershock, volcanic ash spread alarm in Mexico

A magnitude 6.2 aftershock that shook Mexico on Saturday was blamed for five deaths, spreading fear among anxious residents reeling from a string of natural disasters and interrupting the search for survivors from a bigger tremor earlier this week.

Search and rescue workers look for a safe spot as a powerful aftershock rattles quake-hit Mexico. (AAP)

A strong new earthquake has shaken Mexico, toppling already damaged homes and a highway bridge and causing new alarm in a country reeling from two even more powerful quakes this month that together have killed nearly 400 people.

The Popocatepetl volcano south of Mexico City sent a column of ash into the sky, capping an intense period of seismic activity including two powerful tremors this month that have killed more than 400 people and caused damage of up to $8 billion.

Mexico’s capital was shattered by Tuesday’s magnitude 7.1 quake that flattened dozens of buildings and killed at least 307 people. The government’s response to the disaster is under close scrutiny ahead of a presidential election next year.

Although the latest quake was not as destructive, fear is running high among the population. Terrified residents ran into the streets, where they crouched and prayed as earthquake sirens went off. Two women died of heart attacks as the ground shook, the city government said.

The US Geological Survey said the new, magnitude 6.1 temblor was centred about in the state of Oaxaca, which was the region most battered by a magnitude 8.1 quake on September 7.

US Geological Survey geophysicist Paul Caruso said the new temblor was an aftershock of the 8.1 quake, and after a jolt of that size even buildings left standing can be more vulnerable.

Buildings swayed in Mexico City, where nerves are still raw from Tuesday's magnitude 7.1 temblor that has killed at least 305 across the region. Many residents and visitors fled homes, hotels and businesses, some in tears.

At the Xoco General Hospital, which is treating the largest number of quake victims, workers ordered visitors to evacuate when seismic alarms began to blare.

As rescue operations stretched into day five, residents throughout the capital have held out hope that dozens still missing might be found alive.

More than half the dead - 167 - perished in the capital, while another 73 died in the state of Morelos, 45 in Puebla, 13 in Mexico State, six in Guerrero and one in Oaxaca.