More than 60 children have died due to lack of oxygen at a hospital in India because suppliers' bills have not been paid, it has been claimed.

Parents were forced to watch the young patients die on the wards at the hospital in Gorakhpur, in Uttar Pradesh, after a disruption to the oxygen supply.

Now Yogi Adityanath, the chief minister of the state and a key ally to Prime Minister Narendra Modi, faces calls to resign.

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.

They claim the company that supplies oxygen to the hospital had earlier threatened to stop distribution unless the government paid its long-overdue bill of 6.8million rupees (£80,000).

Medical staff attend to a child at the Gorakhpur hospital where parents blame many deaths on oxygen shortages

An child suffering from Japanese Encephalitis lies on a bed at The Baba Raghav Das Medical College in Gorakhpur

At least 64 children died over six days at the hospital, with Indian media reporting that 30 deaths on 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.

A father mourns the death of his child outside Baba Raghav Das Hospital in Gorakhpur amid allegations an upaid bill led to a disruption in oxygen supply

Gorakhpur is the hometown of Uttar Pradesh chief minister Yogi Adityanath, a key ally of Prime Minister Narendra Modi

Chief Minister of Uttar Pradesh (UP), Yogi Adityanath walks out after a visit to the Baba Raghav Das Hospital in Gorakhpur, in the northern Indian state of Uttar Pradesh, on August 13, 2017

'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.'

Gorakhpur'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.

A father cares for his child at the Baba Raghav Das Hospital where at least 36 children have died in the past six days

An Indian relative holds the body of a child while walking out of Baba Raghav Das Hospital in Gorakhpur, in the northern Indian state of Uttar Pradesh, on August 13, 2017

Relatives mourn the death of a child at Baba Raghav Das Hospital, where angry relatives have demanded answers

'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.