More than 40 children and a dozen of adults have died while hundreds are admitted in the hospitals of the eastern part of the state, officials said.
The rapid spread of the disease has led Uttar Pradesh chief minister Yogi Adityanath to order an urgent increase in bed capacity at hospitals. He has also asked the district health department to conduct a detailed investigation into the cause of the fever.
Patients have been complaining of the symptoms of high fever, joint pains, headaches, dehydration and nausea. In some cases, the patients are also suffering decline in platelet counts and rashes on the body.
Those who died of the disease tested negative for Covid-19 and no link has been found with the ongoing pandemic.
Ruling Bharatiya Janata Party (BJP) lawmaker Manish Asija from Firozabad, one of the worst-affected regions, said on Monday that 40 children have died in the past few days in the district alone.
Mr Asija called the situation alarming and said the reasons could be "water-logging, lack of sanitation and hygiene are the reasons behind the disease spread."
Cases have been reported from Firozabad, Agra, Mathura, Mainpuri, Etah, and Kasganj districts of the state.
Several physicians suspect it to be dengue fever, a mosquito-borne viral infection that is widespread in southeast Asia. But the state government has denied its link to dengue as well.
Mr Adityanath said that the first cases with the illness was detected on 18 August, adding that teams would be formed to conduct a probe.
“Due to lack of awareness at the local level, the patients were taken to private hospitals and clinics. After learning about the fever, the health department and administration appraised authorities of the situation at the state level,” the chief minister told reporters.
"Directions have been issued to ensure adequate manpower at medical colleges," he said.
Scenes at Firozabad medical college showed rows of children in the hospital wards accompanied by worried parents.
Family of a six-year-old boy, who succumbed after three days of high fever, told NDTV that he was not able to make it to the hospital in Agra. "Ten minutes before we reached Agra, he breathed his last," he said.
The deaths have also been linked to scrub typhus, according to a report by Live Hindustan. The samples taken from Koh village of Mathura were found infected with scrub typhus.
Scrub typhus, also called as bush typhus, is caused by a bacteria that passes through bites of infected viral mites. The symptoms begin within 10 days of being bitten, similar to vector-borne diseases. The symptoms are fever and chills, headache, body aches, rashes and muscle pain.
In 2014, several children were detected with antibodies to these scrub typhus bacteria in Gorakhpur when several children died in the city.
Several schools were ordered to remain shut in the affected areas by the authorities even as education institutions are opening up in several parts of the state after a long hiatus due to the pandemic.