"The terrorists have used a humvee in their attack. They have targeted the National Directorate of Security unit in Ghazni city," he said, adding that 40 people were wounded.
The interior ministry in Kabul and a health official in Ghazni also confirmed the car bomb.
Afghan army soldiers arrive at the site of a car bomb blast. File photo
Taliban spokesman Zabihullah Mujahid said on Twitter that its insurgents had carried out the attack.
The bombing comes a day after President Ashraf Ghani and his rival Abdullah Abdullah signed a new power-sharing accord in Kabul, ending their bitter months-long feud.
The agreement overcomes one of the hurdles to negotiations with the Taliban, which warned Monday that talks cannot open until a piecemeal prisoner swap is completed.
"That which is taking place in Kabul is only a repetition of the past failed experiences," Suhail Shaheen, a Taliban spokesman, said on Twitter in what was the group's first reaction to the Ghani-Abdullah deal.
"Afghan sides should focus on real and sincere solution to the issue... The prisoners' release process should be completed and the intra-Afghan negotiations should start."