KARACHI (AFP) - Four infants died on Friday at a private hospital in the southern Pakistani city of Karachi after oxygen supply to incubators was disrupted following a power failure, police said.

The deaths occurred at the privately run Shah National Children Hospital in the low-income Korangi district of the southern Pakistani megacity.

"The children were on incubators and apparently the power failure disrupted their oxygen supply resulting in the deaths of four children," senior police officer Nazir Mirbahar told AFP.

A father of one victim described how he had brought his three-month old daughter to hospital only the night before.

"She was actively playing and rejoicing when I brought her to the hospital because she was suffering from light fever," Mr Aqeel Ahmed said.

Angry relatives stormed the hospital, ransacking equipment and blocking roads with burning tyres and boulders.

The Health Department has started an inquiry into the children's deaths and a report will be submitted to the provincial government within 24 hours.

"We cannot say right now how the deaths occurred but we have formed a two-member committee to probe into the matter," Dr Zafar Ejaz, a senior health official, told AFP.

The police said they had detained some of the hospital staff and were tracing the owner of the facility as part of their investigation.

"Cases of criminal negligence will be registered against the hospital management, if the parents press charges," Mr Mirbahar said.

Pakistan is blighted by rolling power cuts, usually because of "load-shedding" but sometimes caused by people not paying their bills, with government offices among the worst offenders.

Ordinary people struggle without electricity for 12 to 18 hours a day in the blistering heat of summer when temperatures in some places top 50 deg C.

Solving Pakistan's energy crisis was a key campaign pledge for Prime Minister Nawaz Sharif in the run-up to the 2013 General Election.