Gorakhpur - A key ally of Prime Minister Narendra Modi was facing calls Sunday for his resignation after dozens of children died at a government hospital in northern India that suffered oxygen shortages.Yogi Adityanath, the chief minister of Uttar Pradesh state where the deaths occurred, visited the hospital on Sunday as angry relatives rushed to the scene demanding answers.At least 64 children died over six days at the hospital in Gorakhpur, with Indian media reporting that 30 deaths Thursday and Friday were from a lack of oxygen in the children's wards.Suppliers' bills had allegedly not been paid, leading to a shortage that saw panicked families using artificial manual breathing bags to help their stricken loved ones.Local officials have conceded there was a disruption to the oxygen supply at the hospital, but insist the deaths were caused by encephalitis and other illnesses, not a lack of available oxygen.Adityanath, a firebrand Hindu priest from Modi's conservative nationalist party, vowed to leave no stone unturned as he toured the hospital in his signature saffron robes."If the investigation finds any authority guilty of negligence, he will not be spared at any cost," Adityanath told reporters in Gorakhpur, the city he represented for nearly two decades.He repeated that the deaths were caused by encephalitis - a mosquito-borne virus that every year ravages poorer, eastern parts of Uttar Pradesh, India's largest state with more than 200 million people.But parents have recounted panic and horror as their children suddenly began gasping for air amid an apparent drop in oxygen, and nurses handed out manual pumps to aid their breathing."I am a poor man who doesn't understand what happens here, but it was clear that day the oxygen wasn't going up. The doctors and other staff here were very worried," Ram Prasad, sitting by his two-year-old daughter's bedside, told AFP. "They rushed to my kid too and gave us a manual pumping machine. It was the longest one-and-a-half to two hours of our lives. We spent the night pressing that machine so that nothing happened to our daughter."Others described the hospital in total chaos, with helpless parents carrying the lifeless bodies of their children, crying out for help."It was very sudden. We didn't know what was happening," Bechna Devi told AFP beside her three-and-a-half year old daughter Saroj."Every hospital staffer around us was in a rush and they simply told us to use that pump machine for our child."Tragedy of epic proportionsGorakhpur's police commissioner Anil Kumar told AFP on Sunday that 11 more children had died at the hospital on Saturday."But I reiterate, they were not due to lack of oxygen supply," he said.As anger grew, opposition parties and government critics led the charge for Adityanath's resignation."The death of innocent children in Gorakhpur is a tragedy of epic proportions," Sanjay Jha, a spokesman for India's main opposition Congress party, told AFP."The fact that it happened in a state-run hospital is a manifestation of pathetic governance. The buck stops with CM Adityanath, as his government has clearly misplaced priorities... He should resign forthwith owning full moral responsibility."The hospital's day-by-day breakdown of the death toll showed a jump Thursday when 23 infants died, including 14 babies at its neo-natal unit.Doctors admitted that the oxygen supply had been disrupted for a couple of hours late Thursday, but said no deaths had occurred at that time.The head of the hospital was stood down pending an inquiry into the oxygen shortage, which allegedly stemmed from nearly $100,000 in overdue bills, some dating back to November."If there is any pending payment which is yet to be made to any gas supplier, then it should be done immediately," senior state health official Anita Bhatnagar Jain told the Press Trust of India on Sunday."There should be no shortage of oxygen... and adequate stock of oxygen must be maintained."Adityanath, who won Uttar Pradesh in a landslide in March for Modi's Bharatiya Janata Party, ordered a review of oxygen supplies in the state's hospitals and medical colleges.