On his face is an angelic smile, in his pocket a blood-stained 50-rupee note. Ishaq Khan, a 12-year-old schoolboy, was given the money – equivalent to just 40p – to carry a bag to a spot in a busy bazaar in Kohat, a town in the North West Frontier Province of Pakistan.

As he walked away, the bag exploded, throwing him to the ground with a shattered foot and leaving shoppers dead and wounded all around him.

In a macabre new tactic, Taliban militants have begun paying children to plant lethal bombs in Pakistani cities. Ishaq, who comes from a poor family that barely survives on the money his father earns from house-painting jobs, had been working at the Orakzai bus stop of the city’s main Terah Bazaar, earning a few rupees a day by helping people to load lorries and buses.

Two weeks ago a man with a moustache but no beard approached him and offered him the note to leave a blue plastic bag in a crowded area between several shops.

“I was excited to get 50 rupees,” said Ishaq. “That’s more than I earn the whole week.”

He picked up the bag and pocketed the money, enjoying the feel of it as he thought about whether he might spend it on a rare treat such as an ice-cold Coca-Cola, or take it home to his mother. The bag was of plastic sacking of the type used to carry sugar, and was not heavy.

“I put it down, turned back and had not walked 20 steps when there was a big blast and I was thrown,” he said. “I don’t remember what happened then.”

When he woke up he was in Kohat hospital with the other victims of the blast. The 50-rupee note was still in his pocket, covered in blood.

When he learnt that three people had been killed and 23 injured he was horrified. “I never imagined it was a bomb,” he said, his eyes filling with tears. “I move bags for people all day.”

Doctors at the hospital say his left foot has multiple fractures and the heel is completely crushed. Yesterday he had the first in a number of operations needed it if he is to walk again. His back is peppered with shrapnel from the bomb but his family has no money for painkillers.

The hospital has run short of blood supplies because of the bomb, so local medical students rallied round to donate some.

The Kohat bombing was one of a succession of deadly attacks since the Pakistan military launched an offensive against the Taliban in the Swat Valley region. The Taliban has vowed to carry out bombings in Pakistani cities in retaliation. There have been at least 16 attacks since the operation started in late April and more than 100 people have been killed.

Most of the attacks have been in Lahore and Peshawar. The latter city’s only big hotel, the Pearl Continental, was blown up on Tuesday. Militants fired on the hotel guards, drove a lorry laden with half a ton of explosives up to the buildings and detonated it, killing 18 people, including two United Nations officials.

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