"It was a magnetic bomb," said Interior Ministry spokesman Sediq Sediqqi. "It was either attached to the vehicle belonging to the foreigners or it was planted and detonated remotely."
The Taliban insurgents, ousted from power by a US-led coalition in 2001, could not immediately be reached for comment.
The ISAF international military coalition in Afghanistan gave no details of the attack but said in a statement two of its service members were killed in the area on Monday.
A Western security source told Reuters the victims were American. Another source said at least three civilians were also killed.