KARACHI, Pakistan >> A roadside bomb tore through a bus taking Pakistani navy employees to work in Karachi on Thursday, killing three people in the third such attack in the country’s largest city this week, authorities said.

The blast showed the strength and reach of Islamist extremist networks in Pakistan, and came less than a week after the country’s army chief publicly claimed that the “backbone” of the militants had been broken.

Two of those killed were navy employees, while the third was a passer-by, said navy spokesman Salman Ali and Seemi Jamali, a doctor at the city’s Jinnah Hospital. Eleven people were wounded.

On Tuesday, four people were killed in twin blasts against navy buses in the city.

The Pakistani Taliban claimed responsibility for those strikes, and warned of more unless the army stopped its campaigns against their heartlands in the northwest close to the Afghan border. 

Karachi is home to 18 million people and is the economic heart of Pakistan. It is far from the northwest, but has not been spared the Islamist violence wracking the country over the last four years. The Pakistan navy is based in the city, which is on the Arabian Sea.

The army has launched several offensives in the northwest, but bombings against government and security force targets, as well as indiscriminate attacks on public places, have continued. The Pakistani Taliban have little direct public support, but their identification with Islam, strong anti-American rhetoric and support for insurgents in Afghanistan resonates with some.