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The children of a shepherd family were playing in a mountainous village in Khyber Pakhtunkhwa province when they found the hand grenade and accidentally set it off, local government official Zariful Maani told AFP.
The two brothers aged nine and 10 died, while their seven-year-old female cousin is in a critical condition, Maani said.
Another local government official, Aizaz Ahmad, confirmed the details.
The village is close to the Swat Valley, where the army sent 30,000 troops in 2009 to battle Taliban fighters led locally by cleric Maulana Fazlullah.