The airstrike also came a day after suicide attacks in the capital, Damascus, killed at least 30 people.
Al Jazeerareports that the mosque, located in the western countryside of Aleppo, was struck during the evening prayer when the building was full of worshipers.
Just last week a Pentagon investigation into a USA attack on Yemeni village which killed dozens of women and children absolved military personnel of any war crimes.
Civil defense team members try to rescue people trapped under the debris of a mosque after an aerial attack in Aleppo on March 16.
The Pentagon provided a black-and-white image depicting what appeared to be an old mosque with a flattened building a short distance away.
"We did not target any mosques", John J Thomas, a spokesman for US Central Command, told the New York Times.
People are leaving Aleppo and fleeing to Idlib as the former has been witnessing fierce battles between the rebels and the Islamic State group around al-Bab, NBC reported.
A U.S. spokesperson told Airwars, a group that monitors coalition airstrikes in Syria and Iraq, that the target had been "assessed to be a meeting place for al Qaeda, and we took the strike".
Russian Federation is expecting the Pentagon to comment on the deadly airstrike on a mosque in Syria's Aleppo province, where at least 40 civilians were feared killed overnight into Friday, March 17.
"This [strike] hit its intended target", Capt. Jeff Davis said.
Jihadi and militant websites said the group, which aims to encourage people to adhere to Islam by peaceful means, was holding a weekly meeting with about 250 people in attendance.
Syrian and Russian military have also carried out many airstrikes in Idlib and Aleppo provinces during the war.
Since then, more than 320,000 people have been killed and 11 million displaced in the conflict, according to activists.