At least three children were killed and two others critically injured in a blast on Wednesday in Pakistan's northwest Swat, the media reported.According to Geo TV, the blast took place when the children were playing with a mortar shell in a field in Arkot village in Swat, a district in northwest Khyber Pakhtunkhwa province.The police said that the explosion was triggered when the children started throwing the live mortar shell to each other considering it as a toy.Police said the mortar was fired from an unknown location, which missed its target and did not explode.