At least 40 people were killed and over 130 injured when an explosion tore through a political rally in Pakistan's northwestern Khyber Pakhtunkhwa province on Sunday, police said.

The police said there was no confirmation yet about the cause of the blast that took place at a gathering of the conservative Jamiat Ulema Islam-Fazl (JUI-F) party, known for its links to hardline political Islam, in the former tribal area of Bajaur.

"The JUI-F organised a workers convention in Khar town of Bajaur in which 40 people lost their lives and more than 130 were injured," district police officer Nazir Khan told Reuters.

He said an emergency had been declared in the hospitals of Bajaur and adjoining areas where most of the injured were taken.

Pakistan has seen a resurgence of attacks by Islamist militants since last year when a ceasefire between the Tehreek-e-Taliban Pakistan (TTP) and Islamabad broke down.

However, most of the recent attacks have been on security forces and installations, rather than political gatherings.

The TTP pledges allegiance to, but is not directly a part of, the Taliban in western neighbour Afghanistan. Pakistan's security forces say the TTP have sanctuaries in Afghanistan, which the Taliban run-administration there denies.

Afghanistan's administration condemned the explosion in a statement by their spokesman Zabihullah Mujahid.

The TTP are not the only militant group to carry out attacks in the area, which has also been hit by a local chapter of the Islamic State.