The ambush occurred in Shahwali Kot, 25km north of the provincial capital Qandahar on Monday.
Our correspondent was told by Afghan Militia Forces spokesman, Hajji Guraneh, that the attackers used rocket-propelled grenades and that the convoy vehicles were completely destroyed.
The deputy military commander of neighbouring Zabul province, Major General Saif Allah, told Afghan Islamic Press that Taliban regrouping inside Pakistan intended to target US and Afghan forces and destabilise the government of interim President Hamid Karzai.
Saif Allah said a "clean-up operation" to flush out suspected Taliban fighters had been launched in Shahwali Kot and across Zabul, where US and Afghan forces have been waging a massive offensive for the past fortnight.
Since 25 August about 1000 Afghan soldiers, supported by hundreds of US troops and aircraft, have been pounding Taliban and al-Qaida bases in Zabul's Daychopan mountains, 245km northeast of Kandahar.
The offensive is one of the biggest against the Taliban since its government was forced from power by a US-led military coalition in late 2001.
The US military says almost 100 guerrillas have been killed, while Afghan officials have put the toll at around 124. The mountains lie 180km from the border with Pakistan.
In an earlier attack on Sunday, a bomb exploded in Helmand province killing two security guards.
The target was the provinces’s governor, Hajji Shir Muhammad, who escaped unharmed.
Muhammad has been one of the most outspoken critics of Taliban and has been known to threaten villages with airstrikes if he believes locals are supporting former government fighters.