An aftershock with a magnitude of 6.1 has hit Mexico, just days after a devastating tremor left 295 people dead.

The US Geological Survey said it was centred about 11 miles (18km) from the town of Matias Romero in the southern state of Oaxaca.

Mexico's National Seismological Service recorded thousands of aftershocks just in the first nine and a half hours of Saturday, some of which measured 4.0 or greater.

Details on damage or injuries are not immediately clear, but Mexico City Mayor Miguel Mancera said residents were already on edge since last week's quake.

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Image: Buildings in Mexico City were severely damaged by an earlier earthquake

In the latest tremor, buildings swayed and a seismic alarm was set off in the capital where rescuers have been trying to reach people who remain buried in rubble from Thursday's quake.

Those rescuers have been forced to suspend their efforts.

"I heard the alarm and ran downstairs with my family," said Sergio Cedillo, a resident who was watching rescue efforts when the alarm sounded.

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Image: Specialist rescue teams continue to hunt for survivors

Alejandra Castellanos, who was staying in a hotel in the area with her husband, said she was "frightened" when she felt the earth shake for the second time in two weeks.

"I was frightened because I thought: not again," she said.

Southern Mexico was hit on 7 September by a huge 8.1 magnitude tremor.

The total number killed in the first two earthquakes is thought to be more than 400.

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Image: The Ring of Fire, along which 90% of the world's earthquakes occur

The Central American country suffers so much because of its position on a large grid of tectonic plates, on which all the Earth's countries and seas sit.

It also falls on the 'Ring of Fire', a horseshoe shaped area around the edges of the Pacific Ocean, from Australia to the Andes, along which 90% of all earthquakes occur.