Eighteen patients have died in an intensive care unit in the southern Indian city of Chennai after heavy flooding caused disruptions to generators running life-support systems, officials have said.

Health Secretary J Radhakrishnan said on Friday that an inquiry will be conducted into the tragedy at the MIOT International hospital, which took the official death toll in flood-related incidents to 280.

The announcement came as rains eased on Friday, raising hopes that rescue efforts could pick up in Chennai.

Parts of the flat, coastal city in the state of Tamil Nadu remained under as much as 2.5m of water for a fourth day.

Many residents have spent days stranded on rooftops since more than 345mm of rain fell over 24 hours on December 1.

National Disaster Response Force personnel carry an elderly woman rescued from floodwaters in Chennai, India [BABU/EPA]

Despite combined rescue efforts by the military and civilian emergency services, help has yet to reach many areas. Residents were angered by reports that authorities had released water from brimming lakes without much warning.

"The authorities didn't give us adequate information about water being released from a nearby lake. Before we could take action my car had sunk and I had to move to the first floor of my apartment," said V Raghunathan, 60, a manager at an interior design company living in southern Chennai.

The Tamil Nadu public works department said it did issue warnings, but the information apparently did not reach the public because of a breakdown in media and phone communications. The Chennai edition of The Hindu newspaper did not go to press on Thursday, apparently for the first time in 137 years.

Facing criticism for its handling of the crisis, senior Tamil Nadu officials defended the government at a press conference at the state's water-logged headquarters on Friday evening.

They said authorities have so far evacuated 127,580 people. More than half of them from banks of rivers are now sheltered in relief camps and are being treated for fever and infections to prevent an epidemic.

Military helicopters dropped food to residents stranded on rooftops and the defence ministry doubled to 4,000 the number of soldiers deployed to help the rescue effort.

India's fourth-largest city, Chennai has boomed in the 21st century as a centre for automobile factories and IT outsourcing. But trash-filled drains and building on lake beds in the rush to industrialisation and prosperity has made it more prone to flooding.

People stranded in a residential area are rescued by volunteers in Chennai [Arun Sankar K/AP]

Source: Reuters