JALALABAD, Afghanistan — Afghan officials said on Friday that American warplanes killed 16 civilians as they tried to flee an area in eastern Afghanistan controlled by Islamic State militants, but the United States military insisted the dead had been extremist fighters.

Hajji Saz Wali, the governor of Haska Meena District in the southern part of Nangarhar Province, said the victims included women and children, with eight of the dead from one family, and four others from a second. It was the second time since July 24 that an airstrike in that district killed civilians, according to Afghan officials.

The latest victims died Thursday afternoon when the vehicles they were traveling in were hit by American airstrikes believed to be targeting Islamic State militants in the area, Mr. Wali said. It is not known how many were wounded, he added.

A spokesman for the American military in Kabul said that those killed in the airstrikes had been seen loading weapons into a vehicle. “The strike was conducted in the middle of open terrain,” said the spokesman, Bob Purtiman. “There was zero chance of civilian casualties.”