* Record temperatures expected from St. Louis to Washington
* Utilities working to restore power after weekend storms
* Power restoration seen taking several days
* 288 heat records set on Sunday - weather service
By Ian Simpson
WASHINGTON, July 2 (Reuters) - Relentless heat gripped much of the eastern United States for a fourth straight day on Monday, with about 2.1 million homes and businesses without power after violent storms and soaring temperatures killed at least 18 people.
Power companies warned it could take several days to restore electricity completely in some areas as much of the United States sweltered in a heat wave. Two hundred and eighty-eight temperature records were set nationwide on Sunday.
"Above-normal temperatures will continue to affect a large portion of the country from the northern Plains to the Mid-Atlantic over the next few days," the National Weather Service said.
Many areas will see temperatures from 90 degrees Fahrenheit (32 Celcius) to more than 100 degrees (37.7 C), it said in a statement. Excessive heat warnings and advisories remained over much of the mid-Mississippi Valley and southern states.
Severe thunderstorms were possible in Kentucky and Missouri and in the north-central states, the weather agency said.
Emergencies were declared in Maryland, Ohio, Virginia, West Virginia and Washington because of damage from a rare "super derecho" storm packing hurricane-force winds across a 700-mile (1,100 kilometre) stretch from the Midwest to the Atlantic Ocean.
About 2.1 million homes and businesses from Illinois to New Jersey were still without power, with the biggest concentration in the Washington area.
With power lines down across the region, the U.S. government told federal workers in the Washington area they could take unscheduled leave or work from home on Monday and Tuesday.
Two of the largest property insurers, USAA and Nationwide, said they had received more than 12,000 claims in total from the weekend storms. Most were for house damage.
The storms capped a costly June for insurers, which were already facing losses of at least $1 billion from a hailstorm that ripped through Dallas.
DAMAGE TO POWER GRIDS
Thunderstorms and high winds battered eastern North Carolina on Sunday afternoon, causing three more deaths on top of at least 15 from deadly storms and heat in several states.
About 93,000 Commonwealth Edison customers in northeastern Illinois were without power from powerful storms that brought wind gusts of up to 90 mph (145 kph).
Utilities in Ohio, Virginia and Maryland described damage to their power grids as catastrophic.
FirstEnergy utilities in states from Ohio to New Jersey had about 252,000 customers without power.
Pepco, which serves Washington and much of its suburbs in Maryland and Virginia, reported about 229,000 customers without power.
Baltimore Gas & Electric said about 213,000 customers remained affected. Almost 1,200 utility workers from 12 states and Canada are helping restore power or are on their way to central Maryland, the company said.
Storms killed six people in Virginia and left more than one million customers without power. Two people were killed in Maryland, officials said.
A falling tree killed two cousins, aged 2 and 7, in New Jersey. Heat was blamed for the deaths of two brothers, ages 3 and 5, in Tennessee who had been playing outside in temperatures reaching 105 (41 C).
St. Louis reported three heat-related deaths over the weekend. All were elderly and had air conditioners not in use.
AccuWeather, a weather forecaster, said the "super derecho" storm that caused the widespread damage had raced 700 miles (1,260 km) from northern Indiana to the Atlantic coast in 12 hours.
A derecho - Spanish for "straight" - is a long-lasting wind storm that accompanies fast-moving thunderstorms or showers, AccuWeather said. The most powerful derechos are called "super derechos," described by AccuWeather as a "land hurricane."