The Afghan Islamic Press news-agency said Asmatullah Alizai, Kandahar's police general, confirmed the incident.
There are regular attacks on convoys supplying foreign military bases in Afghanistan, but Friday's attack was believed to be the deadliest.
The convoy was attacked on the highway between Kabul and Kandahar, Afghanistan's main southern city, en route to a military base in Zabul province.
On Wednesday the decapitated body of an Afghan truck driver was found dumped at the side of the same highway in Zabul, not far from the scene of Friday's attack.
The head was placed on the body, officials said.
"Having security is of no use unless a massive amount of aid is invested into the community"
Justtone, Nottingham, UK
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Fighters cut off the noses and ears of three drivers supplying US military bases in the mountainous eastern province of Nuristan last weekend. Police blamed the Taliban.
There are several foreign and Afghan groups providing security to private companies in Afghanistan, including those that deliver logistics to military bases and construction projects.
Fighters with the Taliban movement, which was toppled from power in late 2001 in a US-led offensive, target these security guards and other people working for the government and its allies.
The Taliban-led insurgency was its bloodiest last year, leaving more than 4,000 people dead.
The al-Qaeda-linked fighters have vowed a new wave of violence this year and have already carried out a series of suicide attacks.