Hundreds of villages have been flooded in Bahraich, Shravasti, Balrampur and Lakhimpur Kheri districts. Twenty-eight people have so far died, the state government said.
The fury of Ghaghra, Rapti and Sharda rivers has aggravated after neighbouring Nepal released water.
Doctors' leave has been cancelled over fears of an epidemic outbreak due to the flood waters entering residential areas.
More than 17,000 people are staying in relief camps, an official told IANS, adding the National Disaster Response Force (NDRF) has been deployed while the army has been alerted.
Chief Secretary Alok Ranjan said 73 relief camps have been set up.
The state government has also released Rs.51 crore for relief works. It has also announced an ex-gratia of Rs 1.5 lakh each to those who have died in the floods.
The bureaucrat added Sant Kabir Nagar and Ballia areas were now vulnerable to floods.
A district official told IANS that it was after a gap of 37 years that flood water had entered Balrampur area.