The children, all aged between 8 and 11, found the munition in a field on Thursday and brought it to their home in the district's Azam Warsak area, the English-language Dawn newspaper reported.
Dawn said the children were playing with the mortar shell when it exploded, killing five of them on the spot. Two others, who were wounded in the blast, were taken to the hospital in Wana, the main town of the district.
The area has seen fierce battles between pro-Taliban fighters and government troops over the years, and the explosive could be a leftover from such clashes.
South Waziristan is believed to be contain sanctuaries for al-Qaeda and Taliban insurgents and is also home to Baitullah Mehsud, top commander of Pakistani militants. dpa yam jh im
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