It was the deadliest day for Nato forces since October 26, when 11 American troops were killed.
The US embassy in Kabul, the Afghan capital, said two civilians were also killed in an attack on a police training centre in Kandahar.
The Afghan interior ministry said three suicide attackers stormed the facility. One detonated his bomb outside the gates, blowing a hole in the wall. The other two ran inside and started shooting; they were killed in a gun battle. Three Afghan police officers were wounded.
One of the two people killed, a police trainer, was a US citizen. The other was a Nepalese man working as a security guard.