The shallow magnitude 5.3 earthquake struck at dawn, with its epicentre 53 km southwest (33 miles) of the city of Jahrom, the USGS said.
Iranian media said the quake measured 5.1.
"Four Afghans living and working on a farm were killed in Saifabad village near the town of Khonj," state TV reported.
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The governor of Fars province, Mokhtar Abbasi, told state TV that rescuers were searching the quake zone for any other victims in the sparsely populated region.
Three injured people from the village of Chartala were taken to hospital but later discharged, the official IRNA news agency reported.
Iran is criss-crossed by major fault lines and has suffered several devastating earthquakes in recent years, including a 6.6 magnitude quake in 2003 which flattened the southeastern city of Bam and killed more than 25,000 people.
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