A newly constructed part of a dam in the Charsadda district collapsed and crops have been destroyed by the raging waters, according to officials.
The UN said it had reports that 5,000 homes were underwater in that area.
Al Jazeera's Kamal Hyder, reporting from Pakistan's capital, Islamabad, said that an emergency situation was developing in the country.
"The scale of the devastation is so large that I don't think that even at the provincial level they will be able to cope with the efforts"
Muhammed Ateeb Siddiqui, Pakistani Red Crescent
"The rain is not stopping, many bridges have been washed away and there are reports that 100-year records are being broken."
Khyber-Pakhtunkhwa province has seen between 250mm and 300mm of rain in the past 36 hours, the highest figure recorded in the last 35 years, according to Pakistan's meteorological department.
"We expect more rains in the next 24 hours focused on Khyber-Pakhtunkhwa, but by tomorrow afternoon the intensity will go away," Qamar Zaman, the department's commissioner, said.
Footage on Pakistani TV showed two elderly men in the Peshawar area, clinging to a fence post as a raging torrent swept over their heads.
Other pictures showed villagers in the northwest wading through waist-high waters flowing down the province's streets.
Mian Iftikhar Hussain, the information minister for Khyber-Pakhtoonkhwa, estimated that about 400,000 people were stranded across northwest Pakistan.
"A rescue operation using helicopters cannot be conducted due to the bad weather, while there are only 48 rescue boats available for rescue," he said.
|Most of the dead were killed as their homes collapsed or after being swept away [AFP]
Muhammed Ateeb Siddiqui, the director of operations for Pakistan's Red Crescent Society, said that rescue efforts everywhere were being impeded by the downpour.
"The scale of the devastation is so large that I don't think that even at the provincial level they will be able to cope with the efforts," he told Al Jazeera from Islamabad.
"The terrain is very difficult and the road accessibility has been blocked. A few bridges have been taken away and in some areas the roads are just giving way."
Monsoon season often leads to widespread flooding in Pakistan, which has many low-lying villages.
About 70 people were killed in flash floods in the southwestern Baluchistan province last week, which also uprooted nearly 100,000 people.
The ongoing rain and bad weather are also suspected factors in Wednesday's plane crashnear Islamabad that killed all 152 passengers on board.