Zardari is widower of former prime minister Benazir Bhutto, killed in a suicide gun and bomb attack in the city of Rawalpindi on Dec. 27.
Ejaz Durrani, a spokesman for Zardari in Karachi, said the assailants ambushed the security officer, Khalid Shehanshah, outside his house in Karachi's upmarket Defence neighbourhood.
Zardari was in Islamabad when the attack took place.
"The attackers got out of a white car and opened fire as Shehanshah was standing outside his house," provincial interior minister Zulfiqar Mirza told reporters.
Shehanshah had been among Bhutto's security escort on the day she was killed.
Mirza declined to speculate who could have been behind the Shehanshah's killing, saying: "We will try to investigate from all angles."
"First there were bomb blasts in the city and now they have killed someone who was very close to our party," he said, alluding to a series of small blasts in Karachi earlier this month.
Zardari returned to Pakistan at the weekend after a long stay with his daughters in Dubai, and there has been strong speculation that one reason for his absence from Pakistan was because of death threats against him.
ReutersLast Mod: 22 Temmuz 2008, 15:48