Fears for Middle Eastern Big One after quake strikes fragile fault line in Iran
The quake struck at dawn, with its epicentre 33 miles of the city of Jahrom, the USGS said.
State TV reported: "Four Afghans living and working on a farm were killed ... (in) Saifabad village near the town of Khonj."
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 line and has suffered several devastating earthquakes in recent years.
It included a 6.6 magnitude quake in 2003 which flattened the southeastern city of Bam and killed more than 25,000 people.
Experts have previously warned that hundreds of thousands of people could die if a powerful earthquake one day struck Tehran.
Bahram Akasheh, who at the time was a professor of geophysics at Tehran University and a government adviser, said: "The building codes are almost universally ignored in Iran and Tehran is especially vulnerable to quakes because there is a major fault line running across it.
"The ground conditions in parts of Tehran are unfavorable: too soft, too brittle and too dangerous to build on. Rules are ignored."
Northern Tehran sets of a major fault line that is about 47 miles long and has 100 smaller fractures, Mr Akasheh said.
He said: "The destruction to Tehran would be immense. About 80 percent of the buildings would be damaged or destroyed.
"Tehran is not ready for a big one."
Birth Pangs: November 2016