The quake occurred at a shallow depth at around 05:10 am local time (00:40 GMT), with its epicentre about 29 kilometres north of the city of Herat, the United States Geological Survey said.
Volunteers and rescuers have been working since Saturday in what are now last-ditch attempts to find survivors from the earlier series of earthquakes, which levelled entire villages and affected more than 12,000 people, according to UN estimates.
Local and national officials have given conflicting counts of the number of dead and injured from the previous earthquakes, but the disaster ministry has said 2,053 people died.
«We can't give exact numbers for dead and wounded as it is in flux,» said disaster management ministry spokesman Mullah Janan Sayeq.
There were no immediate reports of new casualties after Wednesday's quake, which hit near Herat city, home to more than half a million people.
The earlier earthquakes completely destroyed at least 11 villages in Herat province's Zenda Jan district, according to the UN.
«Not a single house is left, not even a room where we could stay at night,» said 40-year-old Mohammad Naeem, who told AFP he lost 12 relatives, including his mother, after Saturday's earthquakes.
«We can't live here anymore. You can see, our family got martyred here.