Eight people were missing, either washed away by floods or buried in landslides, while 12 others were injured and being treated in hospitals, police spokesman Dan Bahadur Karki said on Sunday.
"Rescue workers are trying to clear the landslides and open the roads," Karki told Reuters, adding heavy equipment was being used to clear debris.
In southeast Nepal, the Koshi River, which causes deadly floods in the eastern Indian state of Bihar almost every year, was flowing above the danger level, a district official said.
"The flow of Koshi is rising and we have asked residents to remain alert about possible floods," Bed Raj Phuyal, a senior official of Sunsari district where the river flows, told Reuters.
Several rivers have overflown, flooding roads and inundating houses in hill-ringed Kathmandu. (AP PHOTO)
He said the water flow in Koshi River on Sunday was more than double its normal flow.
Authorities said all 56 sluice gates of the Koshi Barrage had been opened to drain out water compared with about 10 to 12 during a normal situation.
Authorities said the flows of Narayani, Rapti and Mahakali rivers in the west were also rising.
In hill-ringed Kathmandu, several rivers have overflown their banks, flooded roads and inundated many houses.
Local media showed people wading through waist-deep water or residents using buckets to empty their houses.
At least 50 people across Nepal have died in landslides, floods and lightning strikes since mid-June when the annual monsoon rains started.
Hundreds of people die every year in landslides and flash floods that are common in mostly mountainous Nepal during the monsoon season which continues through to mid-September.
In the northeast Indian state of Assam, floods have killed dozens and displaced thousands of people in the past few days.
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