By Ben Farmer
Would-be attackers Ghulam Farooq, left, and Mohammad Yunis sit together at the Kabul Juvenile Rehabilitation Center in Kabul. (Photo: AP)
Police seized a nine-year-old and three others aged between 12 and 14 as they tried to cross into eastern Afghanistan.
A spokesman for the Afghan intelligence agency said the boy is believed to be the youngest suicide bomber ever intercepted, though children as young as five have been used to plant bombs in Helmand.
The use of child suicide bombers to target Nato troops, government officials and those working with Hamid Karzai's government has been an insurgent tactic for several years, he said, but had increased recently.
The tactic, coupled with an increase in indiscriminate roadside bombs, had helped push the death toll among children in the conflict to an average of one-a-day, according to a United Nations report published earlier this year.
The child bombers are often lied to, brainwashed, or coerced inside Pakistan and have been responsible for several deadly bombings in recent months.
They are considered easier to recruit and more gullible than adults.
More than 25 failed suicide bombers aged under 18 are currently in Afghan juvenile custody, the spokesman said.
The boys had been given amulets containing verses from the Koran which they were told would protect them from the explosion. On their return, they would be hailed as heroes and their parents would be guaranteed entry into heaven.
Ghulam Farooq, the nine-year-old, said: "Our mullah told us that when we carried out our suicide attacks, all the people around us would die, but we would stay alive."
Lotfullah Mashal, spokesman for the National Directorate of Security (NDS), said the four had been captured at the Torkham border crossing trying to pass into Nangahar province.
All were from the city of Attock, rather than in the turbulent border agencies which have previously been the most fertile recruitment ground.
He said: "In the past we have had suicide attackers from Waziristan and Bajaur, but this is the first time we have arrested child suicide attackers from the settled areas of Pakistan ... They are brainwashed and shown films on mobile phones about supposed atrocities.
"They were told there were rapes and the Koran was being burned by Americans. They were told Kabul had become a Western city, there was no Islam there and whoever went to blow himself up there would go to heaven."
The Taliban have denied using child fighters or bombers and last week issued a statement, saying the accusations came from "propaganda outfits of the enemy".
The statement said the movement's code of conduct did not allow "teenagers (boys with no beards)" to join their ranks.
Afghan officials said the most recent child suicide attack killed four civilians and wounded 12 others when a 12-year-old blew himself up in a bazaar in the Barmal district of Paktika.
- A Royal Marine has been killed by an improvised explosive device in Afghanistan, the Ministry of Defence confirmed on Sunday. The marine, from 42 Commando Royal Marines, based in Plymouth, was involved in an operation to search a compound in the Loy Mandeh Wadi area of the Nad-e Ali district of Helmand province, an MoD spokesman said.