A strong earthquake has struck south of Mexico's capital, Mexico City, causing buildings to shake and prompting workers to rush out on to the streets.

The quake had a magnitude of 5.6, according to the United States Geological Survey (USGS).

The USGS said the quake had hit 240km (150 miles) south of the capital. There were no reports of injuries.

Mexico is currently battling an outbreak of the swine flu virus, which has claimed as many as 149 lives there.

The tremor struck at about 1145 local time (1645 GMT) and was centred near the southern city of Chilpancingo.

"I'm scared," Sarai Luna Pajas, a 22-year-old Mexico City resident, told the Associated Press news agency.

"We Mexicans are not used to living with so much fear, but all that is happening - the economic crisis, the illnesses and now this - it feels like the Apocalypse."

The USGS initially measured the earthquake's magnitude at 6.0 but later revised the figure down to 5.6.