The men, who worked as labourers in the Korangal area of Kunar province in eastern Afghanistan, were killed on Thursday as they drove home from work, Abdul Saboor, Kunar's deputy police chief, said on Friday.
"This was a shocking attack against these poor people," Saboor said.
The assailants stopped the workers' car, searched them and took about $6,000 before gunning them down, Salehzai Didar, Kunar's governor, said. Two workers escaped, he added.
On Friday, in Khost, also in eastern Afghanistan, a suicide bomber killed an Afghan police officer and injured seven other people just hours after US-led forces and Afghan troops killed one Taliban suspect and captured four others in the same province.
Military commanders, including head of the 31,000-strong alliance force in Afghanistan, British General David Richards, are calling for more troops.
Nato estimates more than 200 people have died in suicide bombings so far this year, compared with about 50-60 last year.
Fighting this year is the worst it has been since the Taliban were forced out in 2001 and more than 3,000 people, including more than 150 foreign soldiers, have been killed in the violence.
Using tactics similar to fighters in Iraq, the Taliban and other insurgents are increasingly targeting the poorly trained and equipped Afghan military and police, as well as provincial and district officials and other government workers.
The Netherlands will send a total of 330 soldiers to southern Afghanistan in the coming weeks, boosting its presence there to 1,730 troops, the government said on Friday. But France said it was reviewing its deployment of 200 commandos.
Speaking in Washington, Michele Alliot-Marie, France's defence minister, declined to confirm a French newspaper report which claimed that Paris planned to pull its troops out of southern Afghanistan next year but said it was time to take a fresh look at the deployment.
Canada, which has a major force in the south of Kandahar province and has taken heavy casualties, is increasing pressure on its Nato allies for more soldiers, saying it cannot maintain its 2,300-strong mission without help.
Several European Nato members have troops in more peaceful parts of Afghanistan but restrict the missions the soldiers can carry out or refuse to send them to the more dangerous south.