At least 40 people have been killed in a suicide bombing in the north-west of Pakistan.Officials say the blast struck a security convoy in the Shangla district. Militant attacks have intensified over the past week as the army gets ready to launch an offensive next to the Afghan border. At the weekend the Taliban claimed responsibility for a raid on an army base near Islamabad. Commandos stormed the military complex on Sunday freeing 22 people. Three captives and four militants died in the operation.