Hajji Saz Wali, governor of Haska Meena district in the southern part of Nangarhar province, said the victims included women and children; eight were from one family, and four others from a second family.
"We are deeply concerned by the harrowing reports of civilians being killed and atrocities being committed", UNAMA chief Tadamichi Yamamoto said.
This comes as reports emerged earlier suggested that several civilians were killed or wounded during an airstrike conducted by the U.S. forces in Haska Mina where militants loyal to ISIS terrorist group and Taliban are actively operating.
At least 13 civilians, including 10 children, have been killed by a mortar shell in the Dawlatabad district of northwestern Faryab province, officials said on Saturday.
"The victims were beyond recognition, and they were placed inside the sacks and were buried late last night", Wali added.
"United States Forces-Afghanistan takes all allegations of civilian casualties seriously; reviewing each case fully".
Several districts of Nangarhar are infested with IS militants, US and Afghan officials said. Attaullah Khogyani, a spokesman for the Nangarhar provincial governor, said an unknown number of civilians had died but was unable to say who carried out the strike.
"In the strike, five insurgents were killed and two others were wounded", he told AFP.
He reiterated that US forces take all allegations of civilian casualties seriously and review each case fully, as well as take precautions in a way to minimize harm to Afghan civilians.
In June, at least three Afghan border police officers were killed in United States airstrikes in Helmand province. "For the time being, we do not have a precise figure about the casualties", he told AFP. Back in February, American airstrikes also killed 22 civilians in the same province. Civilian deaths are at an all-time high in Afghanistan.