Some 1.4m storm-struck households in the eastern US are without electricity for a fourth day amid sweltering temperatures.

Three people have already died from heat-related deaths in Maryland, as crews from as far as Quebec and Oklahoma battle to restore power.

Utilities are warning that a number of places could still be without electricity by Friday.

Some Independence Day celebrations have been cancelled near Washington DC.

Traffic lights remain out in many areas of Maryland and Virginia, creating commuting headaches.

Severe storms hit a 500-mile (800-km) area on Friday, knocking out power in parts of US states from Indiana to Delaware.

The extreme weather, which continued over the weekend, has been blamed for more than 20 deaths so far.

With so many households lacking functioning air conditioning or refrigerators, officials are urging residents to check up on elderly or sick neighbours.

High temperatures are expected around the US on Tuesday, including areas not affected by the storms.

The National Weather Service has issued extreme heat warnings for parts of Illinois, Indiana, Missouri, Kentucky, Ohio and Michigan, with temperatures forecast to exceed 100F (38C).

While US Park Service officials said 4 July celebrations in Washington DC would continue as planned, some areas outside the US capital have cancelled fireworks, citing the need to deploy police and fire officials for recovery efforts.

The continued power outages have frustrated residents.

"This has happened time after time and year after year, and it seems as if they're always unprepared," John Murphy, a Burtonsville, Maryland resident, told the Associated Press news agency.

He was waiting on Monday for electricity firm Pepco to restore power to his house as well as his mother and sister's homes nearby.

Another power firm, BGE, said in a statement that crews were working 16-hour shifts on the clean-up and outage effort.

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