Pakistan says its troops fired heavy artillery at Afghan forces at the Khyber Pass border crossing on Wednesday. It was an escalation of violence that came after days of clashes that have killed four people and stranded thousands on both sides.

The two countries have blamed each other for the fighting that broke out on Sunday at their main crossing point. The dispute erupted over the construction of a new border post on the Pakistani side. CCTV correspondent Richard Bestic spoke with former Afghan President Hamid Karzai in London, for his opinion on the conflict.