The men were dragged from their car in the Pashtun Kot district of Faryab province, which borders Turkmenistan, before being gunned down, the provincial police chief Nabi Jan Mullahkhail told AFP.
"They were travelling in a car from (the provincial capital) Maimana to Almar district," he said. "They were chased by Taliban suspects and their car stopped in the Pashtun Kot district."
ACTED (the Agency for Technical Cooperation and Development) released a statement from Paris condemning the killings and clarifying the death toll at six after local officials said seven had been killed.
"They were killed in the course of their work to support the developement in the north. We deplore the deaths of our colleagues while they were carrying out their duties."
ACTED is a non-governmental organisation founded in 1993 that runs reconstruction projects in Afghanistan and across the developing world.
It runs 34 projects in Afghanistan, and last year had 864 local staff and 13 international staff in the country, according to its website.