A bomb on Thursday killed four Afghan security guards working for a private company building roads in the east of the country, the interior ministry said.
The attack capped a week of heavy violence in which eight British and US soldiers have been killed and dozens of insurgents reported dead.
The bomb blew up a small truck belonging to the road construction company in which the guards were travelling in the southeastern province of Paktika, the ministry said in a statement.
"All on board who were martyred were civilian," it said, blaming the attack on "enemies of peace and stability", a term that refers to Taliban and other insurgents fighting the Western-backed Afghan government.
Three British soldiers were killed in a similar bomb blast in southern Afghanistan on Wednesday and another died of wounds sustained in a gunbattle. On Tuesday, another bomb killed four US soldiers.
Last year was the deadliest in an insurgency that has steadily escalated since the 2001 US-led invasion removed the Taliban from government.
US President Barack Obama has approved 17,000 US troops for the country, adding to nearly 70,000 already in Kabul, while NATO has asked its other member nations to also increase contributions.
The US-led coalition announced that its troops, working with Afghan soldiers, killed one militant and detained 10 suspected militants in operations in the east of the country on Wednesday.