The attack took place during the regular Friday bazaar in Kalaya, a town in a Shiite-dominated area of Orakzai tribal district, according to senior local official Khalid Iqbal.

Initial investigations showed it was an improvised explosive device hidden in a carton of vegetables, but officials later said it had been a suicide blast.

“It’s not clear whether the bomber was on foot or riding a motorcycle,” said senior official Ameen Ullah.

 Shahbaz Ali, a resident who was buying food in the market, said he had seen a boy with his face covered ride up on a motorcycle.

“Suddenly a blast took place and then I was unconscious,” he said.

Ullah said 31 people had been killed, including 22 Shiite Muslims. More than 50 people were wounded, with 17 of them in critical condition, he said. Tribal police confirmed the death toll.

Many of the wounded were taken to a hospital in Kohat, a city in neighboring Khyber Pakhtunkhwa province, where doctors said all staff had been summoned to the trauma center.

The blast occurred outside the door of a religious seminary just hours after three attackers tried to storm the Chinese consulate in the southern city of Karachi, killing two policemen. The two attacks do not appear to be connected.

Abbas Khan, the assistant commissioner of the district, said that three members of the minority Sikh community in Kalaya and two security officials were among the dead.

Orakzai is one of seven restive semi-autonomous tribal regions on the Afghanistan border, an area that has long been a focal point in the global “war on terror” and was famously described by Barack Obama as “the most dangerous place in the world”.

Washington has insisted that the mountainous region provides safe havens to militants including the Afghan Taliban and al-Qaeda – an allegation that Islamabad denies.

Pakistan, which joined the US “war on terror” in 2001, says it has paid the price for the alliance.

It has been battling Islamist groups in the tribal belt since 2004, after its army entered the region to search for al-Qaeda fighters who had fled across the border following the US-led invasion of Afghanistan.

Multiple bloody military operations have been carried out in the area, officially known as the Federally Administered Tribal Areas (FATA) and home to about 5 million ethnic Pashtuns.

Security has improved in the region in recent years, though lower-level attacks are still carried out with devastating regularity, often targeting Shiites in the area. It remains notorious for the availability of cheap guns and smuggled goods, along with narcotics including hashish and opium grown in both the FATA and neighbouring Afghanistan.

Residents also complain of undue harassment by security forces, citing disappearances and extrajudicial killings.

Earlier this year, Pakistan passed legislation which paves the way for the tribal areas to be merged into neighboring Khyber Pakhtunkhwa province, bringing them into the political mainstream.

The move will impose the Pakistani judicial system on a region still loosely governed by British colonial-era laws and the tribal system of honor, and which has always existed on the fringes of the state.

The GEO television channel showed footage of military officials cordoning off the bomb site.

In a Twitter post, Pakistan’s minister for human rights Shireen Mazarin condemned the attack and said the casualty toll could rise.

She tied the incident to the situation across the border, saying “as the US fails in Afghanistan,” Pakistan should “be prepared for fallout and we must ensure greater security for our tribal areas.”

The province’s chief minister, Mehmood Khan, also condemned it, saying, “To target innocent citizens is inhumane,” according to the Dawn newspaper’s online edition.