KABUL (Reuters) - Afghan and U.S. military authorities are investigating reports that as many as 13 civilians were killed in an operation in the northern region of Kunduz, officials said on Monday, but details remain sketchy three days after the incident.

Afghanistan's flag flies over the Presidential Palace in Kabul, Afghanistan April 24, 2017. REUTERS/Jonathan Ernst

There were conflicting reports of the casualty numbers in Friday’s incident in the district of Chahardara outside Kunduz. A local council member said 13 people were killed, but a spokesman for the provincial governor said it was just one.

However, the incident underlines the risk of increased civilian casualties from the recent U.S. decision to step up air strikes and special forces operations in Afghanistan to break a stalemate with Taliban militants.

“There is a need for a proper investigation to prove these allegations and we will launch it soon,” Ghulam Qader Akbari, the governor of Chahardara, said on Monday.

On Saturday, the NATO-led Resolute Support mission in Kabul confirmed the incident had occurred in Kunduz and said an investigation had begun, but gave no details.

“We are aware of allegations regarding the potential for civilian casualties as the result of a combined operation in northern Afghanistan,” Resolute Support spokesman Capt Tom Gresback said in a statement.

“Resolute Support takes all allegations of civilian casualties and the responsibility of human life seriously, and is assessing the facts surrounding this incident.”

Khosh Bawar Nasratyar, a member of the local provincial council, said 13 people had been killed by U.S. airstrikes in the district.

But Nematullah Temor, a spokesman for the provincial governor, said only one civilian had been killed and six wounded when coalition forces hit a car taking people to the villages of Ghar O Qeshlaq and Qatl-e-Am to evacuate Taliban casualties.

He said other reports of civilian casualties were “untrue and baseless”.

Afghan Special Forces said 48 insurgents had been killed in Chahardara but denied any civilian casualties, saying the area had been evacuated before the operation.

But graphic pictures purporting to show the aftermath of the incident, and unconfirmed reports that as many as 60 people had been killed, were widely shared on social media, highlighting the risk that public opinion could turn decisively against the government over the issue.

The most recent United Nations civilian casualty figures, released last month, show air strikes drove a rise of 52 percent in the first nine months of 2017, to 205 killed and 261 injured, versus the corresponding period last year.

Former President Hamid Karzai, who clashed bitterly with the United States over civilian casualties when he was in power, condemned the incident, but there was no comment from the government of President Ashraf Ghani.