A magnitude 6.3 earthquake hit western Afghanistan on Saturday morning, the United States Geological Survey said, with the epicenter close to the region's largest city.

The USGS said the epicenter was 40 kilometers (25 miles) northwest of the city of Herat, and was quickly followed by three aftershocks of magnitude 5.5, 4.7, and 6.2.

An AFP journalist in the city said residents and shopkeepers fled buildings when the first quake hit around 11:00 am (0630 GMT), but that there were not yet reports of casualties or structural damage.

"People are worried and terrified. Women, men and children, everyone is out of their homes," he said. "There have been aftershocks. Everyone is scared and tense. No one wants to stay inside."

Hundreds of fatalities were possible, according to a USGS preliminary report.

"Significant casualties are likely and the disaster is potentially widespread. Past events with this alert level have required a regional or national level response," it said.

USGS had earlier reported the shallow quake's magnitude as 6.2.

Herat -- 120 kilometers east of the border with Iran -- is considered the cultural capital of Afghanistan.

It is the capital of Herat province which is home to an estimated population of 1.9 million, according to 2019 World Bank data.