Several NATO troops are among at least seven people killed in a blast in Kabul as violence intensifies in the run-up to Afghanistan’s presidential election.The soldiers military convoy was targeted by a suicide bomber as it drove along a road in the east of the capital. Two UN staff workers are also among those killed in the explosion, which left more than 50 people injured. It was the latest and most deadly of a spate of violent incidents over the past few days. Rockets were fired at the presidential palace and Kabul’s police headquarters, though no one was injured. Elsewhere, in the south of the country, a suicide bomber killed three Afghan soldiers and two civilians at a police checkpoint.