Pakistani and Afghan troops Wednesday traded heavy firing in border region. Dawn TV said that three Afghan troops were killed in the incident.     According to the TV channel two Pakistani soldiers were also injured. IRNA reporter in Islamabad said that no official word is yet available about the incident. Afghan forces shelled a Pakistani checkpoint at the Angor Adda border town in South Waziristan tribal region. It was not clear as to what prompted the clash. Dawn said that it is not clear if NATO forces were involved in the clash. Pakistan have deployed over one hundred thousands troops along the border with Afghanistan to check the cross-border movement of the Taliban militants. There is also a huge presence of Afghan and foreign troops on Afghan side of the border to stop infiltration from the Pakistani tribal regions.  From: Islamic Republic news agency