Several children were among at least six civilians who were killed in a US drone strike targeting a suspected suicide bomber in Kabul on Sunday, international media reported. The US has said it is "investigating" reports of civilian deaths.

Witnesses told Al Jazeera that the airstrike launched to destroy a car said to be laden with explosives killed six civilians, including at least three children.

CNN, meanwhile, reported that nine members of a family, including six children, were killed in Sunday's attack in the crowded Afghan capital, where thousands of Afghans are still trying to flee the Taliban.

A resident of the area where the strike took place said neighbours estimated "up to 20 people" might have been killed, according to the report.

Local media also reported that civilians were killed in the strike.

Different local and foreign media outlets cited varying numbers of casualties quoting residents, while there was no official confirmation from the authorities. But all of them reported that the victims were Afghan civilians and not militants affiliated with the militant Islamic State (IS) group, as the US has claimed.

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The youngest among those killed was a two-year-old girl, a brother of one of the deceased told CNN.

They were "an ordinary family", he was quoted as saying. "We are not ISIS or Daesh and this was a family home — where my brothers lived with their families."

A Taliban spokesman also said the US drone strike resulted in civilian casualties, and condemned the United States for failing to inform the Taliban before ordering the strike.

Spokesman Zabihullah Mujahid told China's state television CGTN on Monday that seven people were killed in the drone attack, describing the US action on foreign soil as unlawful.

“If there was any potential threat in Afghanistan, it should have been reported to us, not an arbitrary attack that has resulted in civilian casualties,” Mujahid said in a written response to CGTN.

'Assessing the results of the strike'

Pentagon officials said the suicide car bomber had been preparing to attack the airport in Kabul, where US troops were in the final stages of a withdrawal from Afghanistan, on behalf of IS-Khorasan, a local affiliate of IS that is an enemy of both the West and the Taliban.

US Central Command said it was investigating reports of civilian casualties from Sunday's strike.

“We are aware of reports of civilian casualties following our strike on a vehicle in Kabul today,” Captain Bill Urban, a Centcom spokesman, said in a statement.

“We are still assessing the results of this strike, which we know disrupted an imminent ISIS-K threat to the airport,” he continued.

“We know that there were substantial and powerful subsequent explosions resulting from the destruction of the vehicle, indicating a large amount of explosive material inside that may have caused additional casualties,” Urban said. “It is unclear what may have happened, and we are investigating further.

“We would be deeply saddened by any potential loss of innocent life,” he said.

The US airstrike came after an IS suicide bomber on Thursday targeted US troops stopping huge crowds of people from entering the airport as they try to flee the new Taliban regime.

Scores of people died in the attack, including 13 US service personnel.

The car that was destroyed by the US strike had been headed for Kabul's airport, a Taliban spokesman had said earlier on Sunday.

About 114,000 people have been evacuated since August 15, when the Taliban swept back into power. The American withdrawal from Afghanistan is due to be completed by Tuesday.