The convoy was attacked as it passed through Saydabad, a district of Wardak province.
A freelance television cameraman filmed what he said was the aftermath of the Thursday attack, about 40 miles from Kabul. The footage showed the burning wreckage of a vehicle on a bend in a mountain road. Militants held up what looked like an M-16 rifle and dragged away a belt of ammunition.
It was not possible to independently verify whether the footage was from the same incident reported by the coalition. The cameraman's name was withheld for his own security.
Fighting between Taliban-led insurgents and security forces continues unabated, despite a nearly seven-year international effort to stabilize the Western-backed government of President Hamid Karzai.
More than 2,000 people have died in insurgency-related violence this year, according to an Associated Press tally, including at least 114 foreign soldiers.
Much of the fighting has taken place in the eastern and southern provinces bordering Pakistan.
Afghan leaders accuse Pakistan of secretly supporting the insurgents and harboring their leaders - a charge Pakistan civilian and military leaders deny.
In the latest and most serious allegation, an Afghan official on Wednesday blamed Pakistan's Inter Services Intelligence agency for an attempt to assassinate Karzai during a military parade in April.
Saeed Ansari, spokesman for the Afghan intelligence service, said the confessions and cell phone records of detained suspects and other unspecified evidence proved the ISI was the "main organizer" of the assassination attempt.
Pakistan's Foreign Ministry on Thursday rejected the allegation as "baseless and irresponsible."
Ministry spokesman Muhammad Sadiq suggested Kabul was trying to divert attention from a "massive intelligence and security failure" of its own.
Confidence in Afghan security forces was further shaken by a June 13 Taliban attack on the prison in the southern city of Kandahar, which freed 400 Taliban fighters.
The Interior Ministry said Thursday it had fired three senior police officials including Kandahar provincial police chief Sayed Agha Saqib and referred his case to the prosecutor's office.
A ministry statement said Saqib was "negligent in his duties," but it did not mention the jail break or what charges he might face.
In other violence, the coalition said warplanes attacked insurgents who fired on Afghan and U.S.-led forces on patrol in the Maywand district of Kandahar province on Wednesday.
The insurgents were killed with "several precision airstrikes," it said.
No government or coalition troops were reported injured.
The coalition also said that a helicopter made a "precautionary landing" in the rocky eastern Afghan province of Kunar on Wednesday.
None of the troops on board suffered serious injury and all returned to their base, it said.
There were no reports of enemy fire forcing the helicopter down.
