Afghanistan has accused Pakistan of targeting a hospital for drug users in the Afghan capital with an airstrike, marking a dramatic escalation of a conflict that began late last month. Pakistan has dismissed the accusation.

Afghanistan's deputy government spokesman said early Tuesday the death toll from an airstrike by Pakistan that hit a hospital treating drug users in the Afghan capital Kabul has increased to 400.

According to security sources, Pakistani troops had “effectively destroyed technical support infrastructure and an equipment storage facility in Kandahar,” the southern Afghan province that is home to...