The U.S. military promptly disputed the U.N. findings, finding fault with the report's verification methods.
The Afghanistan conflict just recorded the most U.S. airstrikes in a single month in nearly a decade.
The US military said it had targeted jihadists in an embattled opposition bastion in the northwest of the country.
A statement said an Afghan unit on the ground had told coalition forces that the area was clear of friendly forces.
U.S. Africa Command says that there have been no civilian casualties as a result of U.S. air strikes targeting al-Shabaab.
President Trump moved to rescind part of an Obama-era order requiring the publication of a report on civilian casualties.
U.S. fighters, bombers, attack aircraft and drones released 7,362 weapons over Afghanistan in 2018.
The airstrike was carried out near an extremist camp near Shebeeley in the Hiran region north of the capital, Mogadishu.
Targets included munition storage facilities, an intelligence site and a military training camp.
The airstrike was in response to an attack against Somali forces by a "large group" of al-Shabab extremists.