KANDAHAR, Afghanistan -- Afghan officials and local residents said Sunday that 22 civilians, mostly women and children, were killed during a joint operation carried out by U.S. and Afghan forces last week in the southern Helmand province.

The presidential envoy for security in Helmand, Jabar Qahraman, said the raid against Taliban insurgents in the Sangin district killed 13 people from one family and nine from another.

"We are saddened to hear the news of civilians being killed," he said. "When the Taliban use civilians as their shield against security forces, such incidents occur."

U.S. Navy Cpt. Bill Salvin, a military spokesman, said "we are working diligently to determine whether civilians were killed or injured as a result of U.S. airstrikes" carried out to support Afghan forces in and around Sangin. The investigation is "continuing and has not reached any conclusions," he added in a written statement.

The UN mission in Afghanistan meanwhile expressed "grave concern" at the violence in Helmand, saying its initial inquiries suggest airstrikes by international forces killed at least 18 civilians, "nearly all women and children."

Hameed Gul, a local resident, said he lost nine members of his family, including his mother and sister, in Thursday's raid. "It's all lie that they were attacking the Taliban," he told The Associated Press in the Helmand provincial capital, Lashkar Gah, where he was staying at the time of the raid.

Kareem Atal, the head of the provincial council in Helmand, said a man, two women and two children who were wounded in the raid have been brought to Lashkar Gah for treatment.

Helmand has seen months of heavy fighting between Afghan forces and the Taliban, who have repeatedly attacked Lashkar Gah. A suicide bomber targeting soldiers in the city on Saturday killed at least seven people.

The Taliban have stepped up attacks across Afghanistan since the U.S. and NATO formally concluded their combat mission at the end of 2014, leaving a smaller contingent of troops behind to focus on training and counterterrorism.

Associated Press writer Rahim Faiez in Kabul, Afghanistan contributed to this report.