At least seven people, including one child, have been killed and 11 injured in a bomb attack near a police van in the country's restive northwest province, officials have said.

The van, which was in Khyber Pakhtunkhwa province's Charsadda district, was being used to provide security for a polio vaccination drive.

Senior police official Saeed Wazir said Wednesday's bomb, weighing 4-5kg, was planted on a bicycle.At least six police personnel were killed in the blast near Sardheri bazaar.

He confirmed it was a remote-controlled IED, and that the van that was targeted was carrying police officers who were heading to various locations to guard health workers during the ongoing polio vaccination drive in Charsadda district.

Zahid Ullah, senior health official at the government-run district hospital at Charsadda, added that there were at least eight people injured in the attack.

The attack comes after three polio vaccination workers were killed in a targeted attack in Karachi on Tuesday.

Deadly attacks

The Pakistani Taliban, other armed groups and al-Qaeda-linked networks all have strongholds in the country's northwest, particularly in the semi-autonomous areas on the Afghan border.

Earlier in the week, a suicide attack on Monday in a market next to Pakistan's military headquarters killed 13 people including eight soldiers and three children.

On Sunday, a suicide bomb in the northwestern city of Bannu killed 26 people, the deadliest attack on Pakistan's military in several years.

As a result of the attacks, Pakistani fighter jets launched an assault on tribal areas near the Afghan border on Tuesday, claiming many fighters were killed in the raids.

Military sources told Al Jazeera that foreign fighters were among those killed in the strikes, including 33 Uzbeks and three Germans.

The Tehreek-e-Taliban Pakistan has waged a campaign against the state in recent years, carrying out a number of bomb and gun attacks.

Pakistan is one of only three countries where the polio virus is still endemic and armed groups have killed more than 30 polio workers and troops protecting them in recent years.

Armed groups oppose vaccinations against polio and consider such campaigns a cover for spying against Pakistan and a conspiracy to allegedly make male Muslim children sterile.

The vaccination campaign is also viewed with suspicion by many in Pakistan after a fake vaccination effort was used as a cover by the CIA in its pursuit of Osama bin Laden.

Source: Al Jazeera and agencies