Pakistan has issued a flood alert for some southern districts, warning of torrential rains, as rescue teams continued searching for missing people in the northwest, where severe floods have killed hundreds over the past week.

At least 21 people have died in monsoon rain-related incidents in Pakistan, authorities said, pushing the nationwide death toll over the last week above 400 as floods and landslides continue to devastate large parts of the country.

Flash floods triggered by cloudbursts in the mountainous northwest have brought destruction since Friday in the worst spell of this year's monsoon.