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.

Rescuers and residents resumed searching on Tuesday for survivors as the death toll from five days of torrential rain rose to almost 400, with authorities warning monsoon downpours would continue until the weekend.

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.

Anguished Pakistanis are searching remote areas for bodies swept away by weekend flash floods as the death toll has reached 277.

Pakistan has received higher-than-normal monsoon rainfall this year, triggering floods and mudslides that have killed more than 540 people since June 26.

Severe monsoonal flooding has caused widespread devastation across Pakistan, destroying homes and sweeping away entire villages.

Pakistani authorities are defending their response to flash floods that killed more than 200 people in a single northwestern district this week.

Pakistan has been grappling with widespread torrential rains in almost all parts of the country that have wreaked havoc and left over 700 people dead and hundreds injured. The deluge crippled the already dilapidated infrastructure, damaging houses, roads, and commercial centres.