US-led forces have admitted civilians were killed in fighting with Taleban earlier this week in the southern Afghan province of Helmand.
"There are confirmed reports of civilian casualties; however, it is unknown at this time how many," the coalition said in a statement.
It said up to 20 wounded villagers had been treated by coalition forces.
Local officials say at least 21 people died when US-led forces called in air strikes against militants near Sangin.
It is the third time in recent months that US special forces, who operate outside Nato remit, have been accused of causing significant civilian casualties.
Public discontent in Afghanistan is growing over the rising number of civilian casualties and the government's failure to improve the lives of most Afghans.
'Human shields'
The US military say they called in air strikes on Tuesday night after US special forces and Afghan troops came under mortar, rocket and small arms fire while on patrol 25km (15 miles) north of Sangin.
The aircraft destroyed "three enemy command and control compounds", spokesmen said on Wednesday. They said troops had killed a "significant" number of militants.
Locals said villages had been hit, and children and women were among the dead.
One villager quoted by Reuters news agency denied Taleban fighters had used them as "human shields".
The US-led coalition statement on Friday said a joint inquiry into the incident was being conducted with the Afghan army.
It said a wounded child had died after being evacuated by the coalition for treatment.
Foreign forces and Afghan troops are trying to drive militants from the lawless, opium-producing region of Sangin.
The rising number of civilian casualties there and elsewhere in the country is putting President Hamid Karzai under increasing pressure.
On Tuesday, the Afghan Senate urged him to open direct talks with local Taleban militants, and for attacks on them to stop.
Earlier that day, the US military had said it was "deeply ashamed" over the killings of 19 Afghan civilians by US Marines in early March.
Correspondents say that casualties in remote battle sites in Afghanistan are almost impossible to verify.
Taleban fighters are often accused of seeking shelter in peoples' homes, leading to civilian casualties, and it is often difficult to determine if people killed in such air strikes were militants or civilians.
Photo caption
US and Afghan troops on patrol