At least six people were killed and more than 15 injured on Thursday when a suspected Taliban suicide bomber rammed his motorcycle into a government bus in Rawalpindi, the military’s garrison city just outside Islamabad.

A senior police officer told Reuters that the bus was carrying workers from the Khan Research Laboratories, a nuclear facility, about an hour’s drive from Islamabad. Another police officer at the scene said a severed head was found which appeared to be that of the suicide bomber.

No one claimed responsibility. But a senior security official told the Financial Times: “This has all the hallmarks of a Taliban type of an attack”.

Pakistan has been hit by a wave of bombings in response to a military offensive against Taliban militants in the northern Swat valley, but Thursday’s was the nearest to the capital since the launch of the offensive.

The attack comes after Rehman Malik, Pakistan’s interior minister, last week told the Financial Times that up to 3,500 militants might have been killed in the Swat campaign. The attack came a day after Yusuf Raza Gilani, Pakistan’s prime minister, claimed victory in the Swat campaign and ordered government officials to step up work for the relief and rehabilitation of up to 2m people who fled the fighting.

A western diplomat in Islamabad said it was too early to claim that the Taliban threat had been repulsed. “The insecurity in Pakistan today will not end with success in one battle. As we go forward, there will be retaliation.”

Pakistan has suffered a growing number of suicide attacks, a tactic that has become popular in the past two years.

As a follow up to the Swat offensive in the past month the military has stepped up its campaign to take charge of the Waziristan region along the Afghan border which is dominated by Baitullah Mehsud, a prominent Taliban militant commander.