At least 900 people are believed to have died in the worst monsoon floods to hit the country in 90 years.
Officials say 19,000 people in the worst-hit areas have so far been rescued.
However, the relief and rescue operation is being hampered as many roads are blocked and communication lines are down.
There are concerns that as more heavy rain is forecast, it could push the flood south to Sindh province.
Major General Attar Abbar, a spokesman for the Pakistani army, said the full scale of the disaster is not yet known.
Thousands of homes and vast swathes of farmland have been destroyed in the northwest, with the main highway to China reportedly cut.
Pakistan's weather bureau said an 'unprecedented' 312mm (12 inches) of rain had fallen in 36 hours in the northwest but predicted only scattered showers during coming days.
The army said it had sent boats and helicopters to rescue stranded people and its engineers were trying to open roads and divert water from key routes.
The European Commission said it had given €30m in humanitarian aid to help the most needy.
In neighbouring Afghanistan, flash floods have killed at least 65 people and affected more than 1,000 families, officials said.
