The demand came in a phone call between el-Sonni and Rosemary A. DiCarlo, UN's Under-Secretary-General for Political and Peacebuilding Affairs.
"In a phone call with USG pol. affairs Rosemary DeCarlo, we asked an immediate investigation by UNSMIL with regards to the bodies found today in Tarhuna hospital, more than 106 civilians including children women many seem to be executed by shots in the head," el-Sonni said on Twitter.
Amin al-Hashemi, a spokesman for the Libyan Health Ministry, said signs of torture were found on most of the corpses, including women and children.
On Friday, the Libyan army liberated the strategic Tarhuna city from Haftar's forces.
Libya's internationally recognized government has been under attack by Haftar's forces since April 2019, with more than 1,000 killed in the violence.
In March, the Libyan government launched Operation Peace Storm to counter attacks on the capital and recently regained strategic locations, including the Al-Watiya airbase and Tarhuna city.
Libya's government was founded in 2015 under a UN-led agreement, but efforts for a long-term political settlement failed due to the military offensive by Haftar’s forces.