The US has said it carried out an air strike in Syria against an al-Qaeda meeting but denied deliberately targeting a mosque in the attack.
Forty-six people were reportedly killed at the mosque in the raid.
The Syrian Observatory for Human Rights said most of those killed in the Thursday evening raid on the village of Al-Jineh, in the northern province of Aleppo, were civilians.
The US has been bombing jihadists in war-torn Syria as part of an international coalition since 2014.
Hundreds of civilians have been unintentionally killed in the country and in neighbouring Iraq during the campaign.
“We did not target a mosque, but the building that we did target - which was where the meeting took place - is about 50ft from a mosque that is still standing,” said Col John J Thomas, spokesman for US central command.
According to a central command statement: "US forces conducted an air strike on an Al-Qaeda in Syria meeting location on March 16th in Idlib, Syria, killing several terrorists."
A spokesman later clarified that the precise location of the strike was unclear - but that it was the same one that was widely reported to have hit the village mosque in Al-Jineh.
“We are going to look into any allegations of civilian casualties in relation to this strike,” he said.
The US-led coalition targeting the Islamic State group in Iraq and Syria did not mention the raid in its daily roundup for Thursday, indicating that the strike was carried out unilaterally by the US.
An AFP correspondent saw rescue workers in white helmets working under spotlights with picks and shovels late on Thursday to dig people out of the rubble.
Much of the building, identified as a mosque by a black placard outside, had been flattened.
The empty prayer hall was covered in debris, and rescue workers stepped through it carefully, deliberating on how to break down a wall to search for more survivors.
Rescuers had earlier left the wreckage site but were forced to double back when they heard moaning coming from the rubble.
“More than 100 people were wounded,” Observatory head Rami Abdel Rahman said on Thursday, adding that many were still trapped under the collapsed mosque.
The village is held by Islamist groups, but the Observatory said no jihadist factions are present in it.
Abu Mohammed, a village resident, told AFP that he “heard powerful explosions when the mosque was hit. It was right after prayers at a time when there are usually religious lessons for men in it.
“I saw 15 bodies and lots of body parts in the debris when I arrived. We couldn’t even recognise some of the bodies,” he added.
Fearing additional air strikes, weekly Friday prayers were cancelled in towns and villages across northern Syria, AFP’s correspondent said.
‘War crime’
The strike was condemned by the Islamist group Ahrar al-Sham, which said targeting mosques was a war crime under international law.
More than 320,000 people have been killed in Syria since the beginning of its six-year civil war.
A ceasefire between government forces and non-jihadist rebel groups was brokered by rebel backer Turkey and regime ally Russia in December, but violence has continued.
The skies over Aleppo province are busy, with Syrian and Russian warplanes and US-led coalition aircraft carrying out strikes.
Russia began a military intervention in Syria in September 2015. In the past, it has dismissed allegations of civilian deaths in its strikes.
The US-led coalition 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.
Guardian service