Chief Minister Nitish Kumar conducted an aerial survey of the flood-affected areas of several districts.
At least four people have died and around 18 lakh were affected in nine districts of Bihar, an official report said.
Five rivers of the state are flowing above the danger level due to torrential rains in the catchment areas bordering Nepal, the report by the Disaster Management Department said.
Chief Minister Nitish Kumar conducted an aerial survey of the flood-affected areas of several districts.
Of the four deaths, the report said, two were reported from Araria, while one each was reported from Sheohar and Kishanganj districts.
A total of 17,96,535 population has been affected by flood waters in 55 blocks of nine districts - Sheohar, Sitamarhi, East Champaran, Madhubani, Araria, Kishanganj, Supaul, Darbhanga and Muzaffarpur.
The report added that about 11 lakh people have been affected in the worst-hit Sitamarhi district, followed by 5 lakh in Araria.
It said 13 teams of NDRF and SDRF have been deployed to carry out relief and rescue operations in the affected districts. The authorities have also opened 152 relief camps giving shelter to 45,053 people, while 251 community kitchens have been made functional.
The chief minister held a high-level meeting to review the flood situation. Later, he conducted the aerial survey of flood-affected areas of Darbhanga, Madhubani, Sheohar, Sitamarhi and East Champaran districts.
Water Resources Minister Sanjay Jha, Chief Secretary Deepak Kumar, Additional Chief Secretary (Water Resources department) Arun Kumar and Disaster Management Department Principal Secretary Pratyaya Amrit accompanied Kumar during the aerial survey.
At the meeting, the chief minister directed the officials to keep decent arrangement for the flood-hit people at the camps. He also ordered for expediting the relief and rescue operations, an official release said.
Around 280 to 300 mm rainfall has been received in the past three-four days in Terai region of Nepal against 50 mm of average rainfall recorded during the past years, it said.
The excessive rainfall created a flashflood in the state, causing five rivers - Baghmati, Kamla Balan, Lalbakeya, Adhwara and Mahananda - to flow above danger level at various places.
The Patna Meteorological Centre has forecast rain with thunderstorm at many places in Bihar in the next four days.