With 2.7 million Texas households still without heat Thursday morning, leaders warned of the dangers with a domino effect on infrastructure.

The lack of power has cut off water supplies for millions, further strained hospitals' ability to treat patients amid a pandemic, and isolated vulnerable communities with frozen roads still impassable in parts of the state.

"This is in many ways disasters within the disaster," said Judge Lina Hidalgo, the top elected official in Harris County, which encompasses Houston. "The cascading effects are not going to go away."

Residents in over 100 counties in Texas have been told to boil their drinking water as treatment plants continue to suffer from energy blackouts, officials said.

Upward of 12 million people in the state -- the country's second largest with a population of roughly 29 million -- either have no drinking water on tap in their homes or have drinking water available only intermittently.

With freezing temperatures expected through the weekend, getting the lights back on will be a slow process, as Texas has lost 40 per cent of its generating capacity, with natural gas wells and pipelines, along with wind turbines, frozen shut.

Hospitals in Houston, the state's largest city, and elsewhere in Texas have reported they have no water. Nearly two dozens deaths have been attributed to the cold snap. Officials say they suspect many more people have died - but their bodies have not been discovered yet.

Dan Petersen, a meteorologist with the National Weather Service in College Park, Maryland, said the storm was pulling out of Texas. "The worst is over and things will be getting better through the weekend," he said.

But freezing temperatures will remain for several more days, Petersen said, and that cold could complicate efforts to quickly get the Texas energy grid back to full capacity.

Governor Greg Abbott told a news conference on Wednesday that he expected a nuclear plant in south Texas to come back online within hours, which along with coal-fired plants' returning to operations should provide enough power for 400,000 homes.

Abbott, a Republican, has demanded an investigation into the management of the Electric Reliability Council of Texas (ERCOT), a cooperative responsible for 90 per cent of the state's electricity.

Critics say ERCOT did not heed federal warnings after a similar cold-weather meltdown in 2011 to ensure that Texas' energy infrastructure, which relies primarily on natural gas, was winterised.

"Every source of power the state of Texas has access to has been compromised because of the cold temperature or because of equipment failures," Abbott said.

Australian Associated Press