President Hamid Karzai condemned the airstrike, which occurred in the Kiwan area of the northeastern Takhar province bordering Tajikistan.
U.S. Secretary of Defense Robert Gates, who met with Mr. Karzai in Kabul Thursday, said the strike killed a senior militant leader. "This is the first I had heard that civilians have been killed, and we certainly will look in to that," he said.
The U.S.-led North Atlantic Treaty Organization coalition said earlier that no civilians were killed in the airstrike. The coalition didn't disclose which country's aircraft carried out the raid. The vast majority of such missions are flown by U.S. forces.
Mr. Gates said the airstrike targeted and killed a "very senior official" from the Islamic Movement of Uzbekistan, or IMU, a militant group formed in Uzbekistan that found refuge in Taliban-controlled Afghanistan in the 1990s and in al Qaeda hideouts in Pakistani tribal areas following the Taliban regime's downfall in 2001. The IMU, which the U.S. has designated as a terrorist organization, returned to northern Afghan provinces in the past year, bolstering the insurgency's strength in what were relatively peaceful areas along the Tajikistan and Uzbekistan borders.
"This was an individual who was responsible for organizing, orchestrating a number of attacks here in Kabul and in northern Afghanistan," Mr. Gates said.
The IMU official, NATO said in a statement, was colluding with the Taliban and was "assessed to be the deputy shadow governor for Takhar province."
In a news conference with Mr. Gates, Mr. Karzai struck a less strident tone than in earlier comments, but asserted that reports he had received said a candidate for parliament and two others were injured and 10 civilians were killed. The Afghan president has been increasingly vocal about civilian casualties inflicted by U.S.-led troops.
Takhar, like many other northern provinces, has a large ethnic Uzbek population. Afghan provincial officials said the parliamentary candidate they said was injured in the bombing, Abdulwahid Khorasani, was Uzbek.
The Takhar governor's spokesman, Faiz Mohammad Tawhidi, said the armed men in the sedan were likely bodyguards for the parliamentary candidate, and no insurgents were found among the casualties. Two jets and two helicopters were involved in the strike, he said.
The coalition said the airstrike killed and injured up to 12 "insurgents" after NATO forces identified several armed men in a sedan that was part of a six-car convoy. Only the sedan was hit, it said.
"We're aware of the allegations that this strike caused civilian causalities and we'll do our best to get to the bottom of the accusations," U.S. Marine Corps Maj. Gen. David Garza, the coalition's deputy chief of staff for joint operations, said in the statement. "What I can say is these vehicles were nowhere near a populated area."
NATO wasn't yet able to deploy a specialized team to investigate the scene of the airstrike. Takhar authorities have already examined the site.
In a separate incident Thursday, two U.S. soldiers died in insurgent attacks in the south and east of the country, according to a NATO spokesman.(WSJ)