The victims were traveling on a road outside the town of Tirin Kot, in Uruzgan province, when the blast ripped through their vehicle, the AP reports.
"This mine was possibly planted by the enemy," it said. Afghan officials refer to the Taliban and other militants as "the enemy."
Militants usually attack foreign and Afghan troops with mines and other roadside bombs, but most of the victims in such attacks have been civilians.
The blast happened a day after two NATO soldiers were killed and three others wounded by a roadside bomb attack on a military convoy in eastern Afghanistan, the alliance said in a statement.
NATO did not disclose the nationalities of the dead and wounded or the location of the attack on Wednesday. However, most of the NATO troops in the east are American.
This year has been the deadliest since the U.S.-led invasion in 2001. More than 6,200 people have been killed in insurgency-related violence, according to an Associated Press tally of figures from Western and Afghan officials.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969