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

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

Wednesday was the deadliest day for civilians in Afghanistan so far this year, with at least 40 people killed, the U.N. said. In the largest attack, a trio of suicide bombers killed 22 people in the busy marketplace of Kandahar city.

NATO has so far said it has no records of civilian deaths from the pre-dawn strike on a house in Logar province. The NATO and Afghan troops were going after a local Taliban leader when the international coalition says they came under fire and called in an airstrike Wednesday.

“This is unacceptable. It cannot be tolerated,” President Hamid Karzai said in a statement condemning the strike in Logar. 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 a thorough 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 U.S. Defense Secretary Leon Panetta visited Kabul.

NATO confirmed only militant deaths from Wednesday’s strike but sent a team to investigate allegations that civilians were killed either along with or instead of insurgents.

“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 Maj. Martyn Crighton, a spokesman for the NATO force in Afghanistan.

Villagers displayed 18 bodies at the provincial capital on 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.

Karzai said in the statement that he was cutting short his trip to China because of the attacks in Logar and Kandahar. He was expected back in Kabul on Friday, said Syamak Herawi, a spokesman for the president.

Nighttime raids on militants taking cover in villages have been a repeated source of strain between the Afghan government, which says the raids put civilians in the crossfire, and its international allies, who say such operations are key to capturing and killing insurgent leaders.

A deal signed in April was supposed to resolve the issue by putting the Afghan government in charge of such operations. But Karzai’s statement put all the responsibility for Wednesday’s strike on NATO.

The conflicting reports about the raid in the Baraki Barak district also show the confusion and strife that continue to surround these raids, despite the document signed in Kabul. Villagers said that the airstrike hit a house where a number of families had come in from out of town for a wedding party. Afghan police said it was a gathering of militant leaders in the area who had holed up in a village house for the night.

Panetta told reporters that he was using his trip to Afghanistan — his fourth to the war zone — to take stock of progress in the war and discuss plans for the troop drawdown. But at a news conference with reporters he focused more on neighboring Pakistan, saying American officials are “reaching the limits of our patience” with the Pakistani government and that the Pakistanis need to do more to root out the al-Qaida-linked Haqqani terrorist network.

Panetta’s explicit and repeated criticism of Pakistan’s inaction, which he also voiced during his visit to India, appeared to signal a somewhat tougher stance and a suggestion that the U.S. is becoming even more willing and quick to strike terrorist targets inside Pakistan.