At least 100 people are dead and at least 500 injured after powerful earthquakes struck western Afghanistan on Saturday, the United Nations said.

The United States Geological Survey reported two 6.3 magnitude earthquakes, as well as several strong aftershocks, near the city of Herat. One of the aftershocks had a 5.9 magnitude.

The initial earthquake struck 40 km west of the city of Herat around 11 a.m. local time, according to the U.N. Office for the Coordination of Humanitarian Affairs in Afghanistan. It was felt in neighboring Badghis and Farah provinces.

"Initial assessments indicate that as many as 100 people have been killed across eight villages in Zindajan Province, Herat Province ... with a further 500 people injured," the agency said in an update, noting that "unconfirmed reports suggest this figure may be closer to 320 people."

The disaster has impacted some 4,200 people -- about 600 families -- so far, the agency said, with 465 houses reported destroyed and 135 damaged. The worst-affected village is Mahal Wadakha, it said.

"Partners and local authorities anticipate the number of casualties to increase as search and rescue efforts continue amid reports that some people may be trapped under collapsed buildings," the U.N. Office for the Coordination of Humanitarian Affairs in Afghanistan said.

In the immediate aftermath of the disaster, the Afghanistan National Disaster Management Authority said at least 15 people were killed and 40 injured.

A number of houses in Herat Province were completely or partially destroyed, the Afghanistan National Disaster Management Authority said, as search and rescue efforts were underway.

The World Health Organization in Afghanistan said on social media it was sending medical supplies to hospitals and ambulances to transport victims "as deaths & casualties from the earthquake continue to be reported."

The U.N. agency shared photos of healthcare workers treating patients outdoors amid the response.

UNICEF Afghanistan also said it was on the ground with its U.N. colleagues "to assess the full impact."

"Once again, children and families in Afghanistan have been affected by a devastating earthquake, this time in western Herat province," UNICEF Afghanistan said on social media.

Last year, at least 1,000 people were killed and another 1,500 injured after a 5.9-magnitude earthquake struck eastern Afghanistan, near the Pakistani border.