KARACHI (Reuters) - Gunmen killed a senior security officer for Asif Ali Zardari, head of Pakistan's ruling party, in the southern city of Karachi on Tuesday, police and party officials said.

Zardari is the widower of former Prime Minister Benazir Bhutto, who was killed in a suicide gun and bomb attack in the city of Rawalpindi on December 27 while campaigning for an election that her party subsequently won.

Ejaz Durrani, a spokesman for Zardari in Karachi, said the assailants ambushed the security officer, Khalid Shehanshah, outside his house in Karachi's upmarket Defense neighborhood.

Zardari, who was in Islamabad when the attack took place, condemned the killing.

"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 was a member of Bhutto's security escort on the day she was killed.

Mirza declined to speculate who could have been behind the Shehanshah's killing, but said it was aimed at destabilizing the three-and-a-half-month old coalition led by Zardari's party.

"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.

(Reporting by Imtiaz Shah and Aftab Borka; Writing by Zeeshan Haider; Editing by Alex Richardson)

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