Friday's attack comes two days after a bomber killed 17 people in the garrison town of Kohat [EPA]

A suicide bomber has blown up a trailer packed with explosives outside a hospital in northwest Pakistan, killing at least 15 people and wounding several dozen, police said.

The bombing on Friday partially damaged a Shia hospital that was under construction in the Pass Kili area of Khyber Pakhtunkhwa province's Hangu district.

"It was a tractor with a trailer a carrying 250 kilograms of explosives hidden underneath materials for construction work at the hospital," Fazal Naeem, Hangu police spokesman, told AFP news agency.

The bomber detonated the explosives at the gate of the hospital, punching a large crater in the ground and partially damaging a nearby mosque run by the same private Shia Muslim trust that operates the hospital, local officials said.

It was the fourth such attack in Pakistan since Monday.

The attack comes just a day after the start of the holy month of Muharram, which is especially important for Shias.

The attack came just two days after a teenage suicide bomber killed 17 people at a busy market in the garrison town of Kohat, which neighbours Hangu.

Rising toll

Around 4,000 people have died in suicide and bomb attacks across Pakistan since government forces raided an extremist mosque in Islamabad in 2007. The attacks have been blamed on networks linked to the Taliban and al-Qaeda.

The northwest has been particularly hard hit. Pakistan last year fought to wrest back control of the northwestern district of Swat from the Taliban and are fighting against homegrown fighters along the Afghan border.

On Monday, a pair of suicide bombers killed 43 people, attacking anti-Taliban militiamen and pro-government elders in Mohmand, part of the tribal belt that Washington considers the global hub of al-Qaeda.

The Pakistani Taliban purportedly claimed responsibility for that attack, threatening death to anyone who forms militias against the group.

On Tuesday, a suicide bomber tried to assassinate the chief minister of Pakistan's southwestern province of Baluchistan, damaging his motorcade and wounding nine people but leaving the minister unhurt.

A purported spokesman for the banned extremist group Lashkar-e-Jhangvi claimed responsibility, saying the official had been targeted for efforts to provide security to Shis Muslims, who are frequently attacked in Baluchistan.

Source: Agencies