There was no immediate claim of responsibility, but the Taliban have vowed to step up attacks on Pakistani security forces in the region to avenge a fresh offensive against Islamist sanctuaries in the tribal district of Mohmand.
"It was a suicide attack. The teenager bomber was on foot and was wearing a school uniform," Abdullah Khan, a senior police officer in the city of Mardan, around 30 kilometres (20 miles) from the regional capital Peshawar, told AFP.
"21 soldiers were killed on the spot and one in hospital. twenty others have been injured," he said. Two military officials in Peshawar confirmed the suicide attack and the death toll.
Mardan is some 50 kilometres east of Mohmand tribal district where Pakistan launched an offensive last month and officials said that a surge in bomb and suicide attacks was a reaction to the operation.
Some 25,000 people have fled in just a week from Mohmand tribal zone, a UN official said last week. 
Pakistan suffers near-daily attacks blamed on Taliban and Al-Qaeda-linked militants that have killed more than 4,000 people since government troops evicted Islamists from an Islamabad mosque in a deadly July 2007 siege.
Local Taliban, who are fiercely opposed to the US-allied government, recently vowed to step up attacks on Pakistani police and security forces.
Pakistan's northwest and tribal areas have been wracked by violence, mostly targeting security officials, since hundreds of Taliban and Al-Qaeda fighters sought refuge there after the US-led invasion of Afghanistan in 2001.
The military has claimed victory in a number of battles against militants, perhaps most notably in 2009 in the Taliban's former headquarters of South Waziristan, but attacks have continued across the country.
Washington has said eliminating militant sanctuaries in Pakistan's tribal belt, particularly North Waziristan, is vital to winning the war against the Taliban in Afghanistan and defeating Al-Qaeda
RAHUL VALLAMBER
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