PESHAWAR, Pakistan - A teenager suicide bomber dressed in school uniform Thursday attacked army recruits at a parade ground in a northwestern Pakistani city, killing at least 31 soldiers, police said.

The deadly attack in Mardan, a main city some 60 kilometers northeast of Peshawar, the provincial capital of Khyber Pakhtunkhwa, also injured 20 recruits.

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Police sources said the attacker walked to the barrack called Punjab Regiment Center at around 8:15 am local time when the recruits were busy in a routine training.

Taliban claimed responsibility for the attack and the group's spokesman Ihsanullah Ihsan warned more attacks to avenge the US drone strikes and operation by security forces in tribal regions.

Taliban militants have launched at least two attacks on the military center in the past.

The police and the army are investigating as to how the bomber sneaked into the heavily guarded training area with boundary walls and guarded round the clock by the soldiers.

Sources said that the bomber may have taken advantage of the relaxed security for a school in the cantonment area and resembled as a student of the school.

Analysts said that the bombing showed that the Taliban have again regrouped themselves and that their suicide network is still functioning despite major operations in the tribal regions and Swat valley.

It is the second major suicide attack in Pakistan's northwest in two months. In December a woman bomber killed over 40 people in the Bajaur tribal region at a World Food Programme distribution center.

Pakistani Prime Minister Syed Yousuf Raza Gilani has condemned Thursday's terrorist attack, reiterating that such cowardly attacks cannot affect moral of the security agencies and resolve of the nation to eradicate terrorism.