AC-130 gunships, along with fighter jets, attacked the convoy
They said 65 people, including a number of tribal elders, had been killed in the attack - an allegation first reported on Friday by the Pakistan-based Afghan Islamic Press.
The delegates added that the Khost convoy had been prevented by armed men from taking the main road, and were following an alternative mountain route when they came under attack. The survivors had returned to Khost.
Local villagers who spoke to a Reuters television crew said the air strikes, staged over seven hours between Thursday night and early Friday morning, destroyed 15 vehicles as well as about 10 houses and a mosque in the village of Asmani Kilai in eastern Paktia province.
One villager, named as Khodai Noor, told the television crew that, after the convoy had been diverted, a hostile local commander had called in US warplanes, saying the cars were carrying members of Osama Bin Laden's al-Qaeda
terror network.
"The people who got hit were going to congratulate Karzai on the transfer of power," he said. "There are no members of al Qaeda or supporters of Bin Laden here."
The Pentagon said US planes attacked the convoy south-west of Tora Bora, the former al-Qaeda stronghold, after it fired on them.
The commander of US forces in Afghanistan, General Tommy Franks, said the Americans were convinced the convoy consisted of leaders of the Taleban or al-Qaeda.
General Franks, also in Kabul for the inauguration ceremony, added that an investigation was under way.
But he defended the pilots' actions, saying: "Friendly forces don't fire surface to air missiles at you. We believe it was a bad convoy. We have reason to believe it was a good target."
Reinforcements
US Defence Secretary Donald Rumsfeld has said that significant numbers of coalition troops are being sent into the Tora Bora cave complex as the search for Bin Laden - the man suspected of masterminding the 11 September terror attacks on the US - continues.
General Franks said Bin Laden had not been sighted for a week.
"There really are only about three possibilities," he told reporters.
"He can be in Tora Bora - or in that area dead - he can be somewhere else in Afghanistan and still alive, or perhaps he may have gotten over into Pakistan."
Musharraf's promise
But Pakistani President Pervez Musharraf, who is visiting China, expressed confidence that Bin Laden had not slipped over the border into Pakistan.
He said there was a "great possibility" that the al-Qaeda leader was dead.
General Musharraf said Pakistan would hand Bin Laden over to the US if he was caught.
"He's not in Pakistan, of that we are reasonably sure," General Musharraf told Chinese state television. "But we can't be 100% sure.
"Maybe he is dead because of all the operations that have been conducted, the bombardment of the all the caves. There is a great possibility that he may have lost his life there."