The attack occurred in the Kohsan district of Herat province as the Halo Trust de-mining team was en route to a mine removal site, police spokesman Abdul Rauf Ahmadi reported.
Halo Trust has been conducting "humanitarian mineclearance" in Afghanistan since 1988, according to the company's website.
"Between 1988 and 2013, HALO Afghanistan destroyed over 766,908 mines (225,908 emplaced mines and 541,000 stockpiled mines), 10+ million items of large calibre ammunition and 45.6+ million bullets."
The Halo Afghanistan team employs more than 3,000 local workers with two resident international staff.
Thursday's attack on the Halo mine clearance workers coincides with the recent suspension of Halo Trust's founder and chief executive officer, Guy Willoughby.
A spokesman for Halo earlier commented on Willoughby's removal earlier this month, saying: "The board of trustees of the Halo Trust has suspended Guy Willoughby as chief executive following a serious deterioration in relations between him and the board over the governance of the charity."
There was no immediate comment from Halo regarding Thursday's attack on its Afghanistan team.