KABUL - The Afghan president said Thursday that 18 people killed in a NATO airstrike in eastern Afghanistan a day earlier were civilians.

The deadly attack has further soured relations between NATO and the Afghan government, which had been already outraged by civilian casualties in previous operations.

NATO said it has no records of civilian deaths from the predawn airstrike in Logar Province. NATO and Afghan troops were going after a local Taliban leader when they came under fire and called in an airstrike, the international coalition said.

“This is unacceptable,’’ President Hamid Karzai said in a statement. “It cannot be tolerated.’’

He criticized NATO for not being able to provide an explanation for the vans piled with bodies of women and children that villagers displayed to reporters.

Karzai’s office said the president had spoken to a man who was related to some of the victims. He promised an investigation and pledged that those responsible would face justice.

Karzai’s condemnation of the strike and NATO’s treatment of it served as a reminder of the ongoing tension between Afghanistan and its Western allies. It came as US Defense Secretary Leon Panetta visited Kabul.

NATO confirmed only militant deaths from the strike but sent a team to investigate allegations that civilians were killed.

“The reason this team has been dispatched down there is because there is such a discrepancy between what our operational reporting indicates and what Afghan officials on the ground are saying happened,’’ said Major Martyn Crighton, a spokesman for the NATO force in Afghanistan.

Villagers displayed 18 bodies Wednesday, including five women, seven children, and six men. Afghan officials said then that some or all of the dead men were militants. Since no government officials have visited the site of the attack, it was not clear if there might be additional dead.