Local officials said the strike was aimed at Taliban fighters and missed. NATO said it was investigating.
Civilian deaths have strained relations for years between the NATO-led military coalition and the Afghan government, and NATO has made efforts to reduce them.
President Hamid Karzai, who has frequently condemned NATO for civilian casualties, called the deaths in Helmand “shocking,” and said in a statement that “NATO and American forces have been warned repeatedly that their arbitrary and improper operations are the causes of killing of innocent people.”
Witnesses said that an unknown number of bombs fell about 11 p.m. Saturday, landing on two family compounds in the Salaam Bazaar area of Now Zad District, a small farming community about 50 miles north of Lashkar Gah, the capital of Helmand Province.
Five girls, seven boys and two women were killed as they slept, the provincial governor’s office said in a statement. An additional six people were wounded.
Grieving friends and relatives drove through the night transporting eight bodies to the provincial hospital in Lashkar Gah, a resident of the village, Haji Janan, said. The other bodies remained buried under rubble as villagers tried to dig them out, he said.
The governor’s office released photographs of men carrying the dusty, bruised bodies of dead children swaddled in sheets into the hospital.
“We brought the dead bodies to show it to the officials, to show that the dead are innocent civilians, not the Taliban,” Mr. Janan said.
Lt. Tyler Balzer, a spokesman for the NATO-led military coalition, said several bombs were dropped but said he could not provide more specifics, including what kind of aircraft were used, until the investigation was complete.
“We are aware of the governor’s claims, and there were airstrikes in the area,” he said. “And right now we have an assessment team on the ground working with the Afghan government.”
Local officials said the airstrike came in response to an insurgent attack on a nearby American Marine base earlier in the night, but that the strike hit the wrong homes.
NATO was also investigating an air assault last week in Nuristan Province that drove out Taliban fighters after they had overrun part of a district center. A joint force of NATO soldiers and Afghan commandos called in airstrikes on Wednesday when they came under fire in the district center of Do Ab. The airstrikes drove hundreds of insurgents out of the town and killed more than 10 of them, NATO said then.
But provincial officials now say that NATO helicopters also killed more than 20 police officers dressed in civilian clothes. Qazi Anayatullah, head of the provincial council, said that as coalition forces arrived, the Taliban fled, leaving their white flags flying over police checkpoints they had overrun. When the officers in civilian clothes re-entered the checkpoints, the Taliban flags were still flying, and NATO helicopters bombed them, he said.
“They mistakenly thought they were Taliban because the police were wearing local dress,” Mr. Anayatullah said. Another local official said the police officers had changed into civilian clothes after the initial Taliban assault, hoping to avoid capture.
Lieutenant Balzer said a NATO assessment team had been in Do Ab for several days. “We’re hoping a clearer picture will come out soon and we’ll be able to release the findings,” he said.
But the episode points to the murky nature of the war and the difficulty of distinguishing between Taliban fighters and armed officers or civilians dressed in traditional garb.
In February, Afghan investigators accused NATO of killing 65 civilians in airstrikes in eastern Afghanistan. But NATO maintained that the people were killed were insurgents, and there were conflicting reports, even among the Afghan investigators, about the number of casualties.
Sharifullah Sahak contributed reporting from Kabul, and an employee of The New York Times from Lashkar Gah, Afghanistan.