US Central Command (CENTCOM) said American airplanes had targetted an al-Qaeda meeting place in the village of al-Jina and that it was not a coalition-led strike.
At least 42 people were killed and dozens more wounded in the attack, the Syrian Observatory for Human Rights said.
Local sources said the strike happened during evening prayers - while the mosque was at its most full.
"The raids by unidentified warplanes targeted a mosque in Aleppo province during evening prayers, killing 42 people, most of them civilians," said the head of the Britain-based Observatory Rami Abdel Rahman.
"More than 100 people were wounded," he said, adding that many were still trapped under the collapsed mosque in the village of al-Jineh, just over 30 kilometres (20 miles) west of Aleppo.
A US 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.”
“It happened to be across the street from where there is a mosque,” said Major Josh Jacques, a CENTCOM spokesperson, adding that the mosque was not hit directly
The village is held by rebel and Islamist groups, but no jihadist factions are believed to present in the area.
Rescue workers struggled to pull survivors from rubble, and dozens of residents were still unaccounted for, the Observatory said.
Footage published by Halab Today, an online media group focused on news in Aleppo, showed piles of rubble where the mosque allegedly stood.
The airstrike came a day after suicide attacks in the capital, Damascus, killed at least 30 people on the sixth anniversary of the start of the Syrian conflict.
More than 320,000 people have been killed in Syria since the conflict began six years ago with anti-government protests.
A cessation of hostilities was brokered by rebel backer Turkey and regime ally Russia in December, but violence has continued across much of the country.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
But the skies over Aleppo province are busy, with Syrian regime and Russian warplanes as well as US-led coalition aircraft carrying out air strikes.
Russia began a military intervention in Syria in September 2015, and in the past has dismissed allegations of civilian deaths in its strikes.
The US-led coalition, meanwhile, has been bombing jihadist groups in Syria since 2014.
The US-led coalition fighting IS said earlier this month that its raids in Iraq and Syria had unintentionally killed at least 220 civilians since 2014.
Critics say the real number is much higher.