PESHAWAR, Pakistan — A Taliban suicide bomber killed the head of a U.S.-backed paramilitary police force battling militants in northwestern Pakistan on Wednesday, one of the highest-ranking security officers ever assassinated in the country.

The bombing in Peshawar that killed Sifwat Ghayur, the head of the 25,000-strong Frontier Constabulary, was the first attack since monsoon rains triggered massive flooding over a week ago. The northwest city often targeted by the Taliban had been calm recently, but renewed violence could further strain a government already struggling with the flood relief effort.

The bomber detonated his explosives next to Ghayur’s car in the center of Peshawar after waiting at a traffic light for the vehicle to approach, said the police chief’s driver, Shakirullah Khan, who was injured in the attack.

“I stopped the car at a traffic light,” said Khan, who was driving Ghayur home from the office. “While my boss was sitting in the left front seat, I saw a young boy move from the sidewalk toward our car and in no time a huge explosion took place and our car was in flames.”

The explosion engulfed several vehicles, killing the police chief and three bodyguards, said Mohammad Haris Khan, a senior police officer. The attack also injured 11 people.

“We have lost a very brave and able official in this cowardly attack,” said Mian Iftikhar Hussain, the information minister for Khyber Pakhtunkhwa province, where Peshawar is the capital.

The Pakistani Taliban claimed responsibility for the bombing, saying they will continue to target government personnel and leaders of a key political party in the northwest who have been outspoken critics of the militants.

“This is our work,” Azam Tariq said by telephone from an undisclosed location. “In the coming days, we will carry out more such attacks against leaders of the Awami National Party, marked people in the government and security forces and also security buildings.”