The killings came as a statement, purportedly from fugitive Taliban leader Mullah Omar, urged militants to step up attacks on Afghan and foreign troops and told NATO to quit Afghanistan.
The ambush victims, who worked for the U.S. military as laborers in the mountainous Korangal area of Kunar province, were killed Thursday while driving home from work, said Abdul Saboor, Kunar's deputy police chief.
In other developments:
Gunmen stopped the civilian workers' car, searched them and took about $6,000 before gunning them down, said Salehzai Didar, Kunar's governor. Two workers escaped, he said.
"This was a shocking attack against these poor people," Saboor said.
Saboor did not identify the attackers, other than to describe them as "the enemy."
Al Qaeda and Taliban insurgents operate in eastern Afghan regions bordering Pakistan.
Afghanistan this year has faced its deadliest surge in violence since U.S.-led forces ousted the Taliban regime five years ago for hosting Osama bin Laden.
The statement from Omar, forwarded by e-mail to The Associated Press by Muhammad Hanif, who claims to speak for the hard-line militia, warned the Taliban would increase their attacks against foreign and Afghan troops in fighting that "would be a surprise to many."
"I would again ask mujahideen (holy warriors) to intensify their attacks but they should avoid any harm to innocent people and children," said the statement, issued on the eve of the Muslim holiday of Eid al-Fitr, that marks the end of the fasting month of Ramadan.
It wasn't possible to verify the statement's authenticity.
The statement said NATO — which has in recent months taken control of security operations across Afghanistan from a U.S.-led coalition — should quit the country. It claimed the alliance was "losing their soldiers only for America."
The statement also alleged that democracy in Afghanistan had failed and the Western-backed government of President Hamid Karzai was seeking an "exit."
"I want to make it clear that the puppet of aggressors and his associates are searching for exit. We will never give them an exit. They will be brought to Islamic justice," it said.
Omar's whereabouts remain a mystery. Karzai told the AP this week that the Taliban leader was hiding in the Pakistani city of Quetta. Pakistan says Omar is Afghanistan.
U.S. and Afghan troops, meanwhile, raided a compound linked to homemade bomb makers early Friday, killing one suspected militant and detaining four others, the U.S.-led coalition said.
During the operation in the eastern Khost province village of Bodakhel, one militant pointed a gun at soldiers, who shot him dead, a coalition statement said. Troops found explosives, detonation cords and multiple blasting caps.
No U.S. or Afghan troops were hurt.
A suicide bomber also attacked Afghan soldiers in the country's east. In Khost's Ismailkheil district, a suicide bomber on foot targeted an Afghan National Army patrol on Friday, wounding five soldiers and three civilians, said Mohammed Ayub, the provincial police chief.