A suicide car bomber targeted vehicles belonging to members of the local police force campaign in Khost province on Saturday, local officials said.
The local police force campaign members fall under the U.S-Forces.
The Ministry of Interior confirmed the incident and said it had been a suicide car bombing at a bus terminal in Khost city.
The incident occurred at about 08:30 am local time.
In addition to the 18 killed, six were wounded including at least two children, the MoI said.
The provincial governor's spokesman Mubariz Zadran said the explosion took place in the Mujahid square area in Khost.
The Taliban has claimed responsibility for the car bomb that rocked Khost city at 8.30am local time Saturday.