According to Uttar Pradesh Police ADG (Law and Order) Anand Kumar, four persons had died in the explosion but the toll could rise further.
SP Shivhari Meena told PTI that according to initial reports "50-60 persons suffered injuries".
Chief Minister Yogi Adityanath, who was away in Mauritius on a three-day official visit, ordered that necessary steps be taken for rescue and relief.
"The chief minister has taken cognisance of the Unchahar accident and has directed principal secretary (home) to ensure that all steps are taken for rescue and relief," principal secretary (information) Awanish Awasthi, who is accompanying Adityanath said.
The explosion triggered panic among the workers who ran helter-skelter. The district administration rushed ambulances to the plant and directed health officials to provide prompt treatment to the injured.