A strong 6.2-magnitude quake shook Mexico on Saturday, causing panic in traumatized Mexico City, where rescuers trying to free people trapped from this week's earlier earthquake had to suspend work.

There were no immediate reports of damage or casualties from the new quake, which was smaller and with its epicenter farther to the south than Tuesday's devastating 7.1-magnitude earthquake that killed nearly 300 people.

However it sparked concern that it could have caused heavy rubble on top of survivors awaiting rescue in Mexico City to shift.

Mexican and foreign rescue crews rapidly evacuated sites they were working at as a seismic alarm went off.

"Oh God, have mercy," exclaimed Teresa Martinez, 74, who had run out in to the street for safety.

"This time we didn't feel the ground move... maybe since the last one we're getting used to them," said Pablo Martinez, who had run out of his central apartment block with his 6-year-old daughter in his arms.

The US Geological Survey registered Saturday's quake at 6.2 magnitude, occurring at 12:53 GMT with an epicenter 19.3 kilometers (12 miles) southeast of the city of Matias Romero, in the southern state of Oaxaca.

The head of Mexico's civil protection service, Luis Felipe Puente, tweeted that "for the moment no effects reported, we are continuing to monitor."

At some Mexico City sites rescue workers were still discussing whether to return to their task of trying to reach survivors of Tuesday's quake believe to be buried under debris in three sites in the city.

Tuesday's quake, which toppled 39 buildings in the capital, was the worst earthquake in three decades to hit Mexico City, following one in 1985 that killed 10,000 people