James Bays, Al Jazeera's correspondent in Afghanistan, said Taliban sources were putting the number killed at about 50, while local sources said it was at least 40.
Bays said Nato had denied any knowledge of the air raid. US forces said they were checking the report.
He said there had been intense fighting in Sangin in recent days.
The claim of civilian casualties is the latest in a string of such incidents which have angered Afghans and prompted Hamid Karzai, the Afghan president, to tell the foreign forces in his country to take more care in battle situations.
The authorities are still investigating an incident in the western province of Herat last month where UN and provincial Afghan investigations have found that about 50 civilians were killed.
The latest casualties came as a US commander admitted that civilian deaths in the country were "shameful".
Colonel John Nicholson, a US brigade commander, apologised on Tuesday to family members of 19 Afghans killed and 50 injured by US forces in an incident more than two months ago.
"I stand before you today deeply, deeply ashamed and terribly sorry that Americans have killed and wounded innocent Afghan people," Nicholson said he told the family members.
"This was a terrible, terrible mistake, and my nation grieves with you for your loss and suffering. We humbly ask for your respect and forgiveness," he said.
There has been growing anger in Afghanistan over civilian deaths in US-led and Nato military operations.