The Pakistan Taliban, an umbrella group of militants also known as Tehreek-e-Taliban Pakistan (TTP), claimed responsibility in a statement sent to Reuters.

The blast hit a truck full of police on their way to guard a polio vaccination drive in the Bajur tribal district, police official Kashif Zulfikar said.

Islamist militants in Pakistan often target polio vaccination teams, believing the immunisation effort is a Western tool to spy on them and make Muslims infertile.

The TTP has been waging a war against the state for years, seeking to overthrow the government and replace it with a harsh brand of Islamic rule.

The militants have ramped up their attacks since they revoked a ceasefire with the government last year.

Political analysts have voiced concern, with some politicians calling for a delay in the elections in the northwest.

A six-man suicide squad drove an explosive-laden truck into a military camp in northwest Pakistan last month, killing at least 23 soldiers, the heaviest death toll in a single attack in years.

(Writing by Asif Shahzad; Editing by Nick Macfie)

By Mushtaq Ali