Flash floods have killed 27 people and affected 70,000 others in northwest Pakistan, where authorities are already grappling with a displacement crisis, government officials and the UN said.

Heavy rain since Sunday has also caused massive destruction to homes, livestock and crops in Mardan and Swabi districts.

"The flash floods inundated five villages in Swabi district, affecting 70,000 people," senior local administration official Shahidullah, who goes by one name, told AFP on Tuesday.

A UN statement confirmed "about 70,000 to 80,000" people had been affected, and said that their sources put the death toll for both districts at 27, with nine people missing and between 400 and 450 homes destroyed.

"As the water swept through villages, the roofs of mud houses collapsed and caused the majority of deaths," the statement released late Monday said.

"The flood waters submerged the cultivated land in these regions and much of the maize, rice, sugarcane and tobacco crops were destroyed. Livestock was washed away and drowned."

Shahidullah told AFP that 13 people in Swabi district were killed.

The floods come as the local government grapples with about two million people who fled a punishing military assault against Taliban militants in the northwest, many of them flocking to camps and family homes in Mardan and Swabi.

Pakistan's military says it has cleared Swat, Lower Dir and Buner districts of insurgents after launching the push in late April, and officials have urged the displaced people to start returning.

So far, about 800,000 people have gone home, leaving 1.2 million still living in host communities and another 165,000 still in camps.

UN officials have confirmed that none of the refugee camps were affected by the floods, as they were already moved in anticipation of the rainy season.