"The head of a security company... was moving with his convoy when he was attacked by a suicide bomber wearing a suicide vest."
The head of the security firm, two of his guards and two young girls were among those killed in the blast, Samon said.
"All were killed on the spot," he said, adding that 24 other civilians were injured.
Earlier on Thursday, the acting governor of eastern Khost province and several senior officials were wounded in an explosion during a district meeting.
The bomb went off in a pile of rubbish outside a meeting hall where Tahir Khan Sabari, the acting governor, was holding talks.
The Taliban later claimed responsibility for the bombing in Khost, which lies on the country's southeastern border with Pakistan.
Hashem Ahelbarra, Al Jazeera's correspondent in Kabul, said: "The governor was holding a meeting with the aim of trying to convince people in Khost to stop using the Pakistani rupee and shift to the local Afghan currency, the Afghani, to boost the economy.
"Fifteen minutes into the meeting a bomb went off."
Mohammad Yaquob, the acting Khost police chief, said: "Behind a wall, which was close to the meeting hall, was garbage in which explosives were detonated.
"The acting governor was slightly wounded on his foot due to broken glass, and the executive director and governor's office manager have been slightly hurt by flying glass, but they are fine.
"There was a big bang but the wall is still standing."
The attack came a day after Kai Eide, the outgoing UN special envoy to Afghanistan, said more emphasis needed to be put on a civilian rather than a military strategy in the fight to defeat the country's anti-government forces.