HOUSTON (Reuters) - Torrential rains have killed at least eight people in Texas and Oklahoma, including two in Houston where flooding turned streets into rivers and led to nearly 1,000 calls for help in the fourth-most populous U.S. city, officials said on Tuesday.

Another 12 people were missing in Texas after the storms slammed the states during the Memorial Day weekend, causing floods and tornadoes that destroyed homes and swept away bridges.

"There are still some significant areas of really devastating flooding in Houston," Mayor Annise Parker said at a news conference, adding she has asked the governor to declare the city a disaster area.

She said most of Houston is high and dry but advised people to stay home.

President Barack Obama said on Tuesday that he had assured Texas Governor Greg Abbott that he could count on help from the federal government as the state recovers from the floods. Abbott has declared a state of disaster in 24Texas counties.

There was no damage estimate available for the state, which has a $1.4 trillion-a-year economy and is the country's main domestic source of energy as well as an agricultural and manufacturing power.

More bad weather was expected with the National Weather Service issuing a flash flood warning on Tuesday forHouston as a line of thunderstorms moved along the Gulf of Mexico coast toward Florida. It said there was a high chance of more rain and thunderstorms for Texas this week.

Rescue workers looked for 12 members of two families missing after their vacation home was swept off its foundation in Wimberley, a town about 30 miles southwest of Austin, where flood waters caused a wave of destruction.

"(People) have lost their homes, they have lost their livelihoods in some businesses," said Wimberley Mayor Steve Thurber.

One of those killed was an 18-year-old girl whose car was swept away by flood waters as she returned home from her high school prom, police in Devine, Texas, south of San Antonio, said.

More than 40 flights were canceled as of 10 a.m. CDT (1100 ET) at airports in Houston and Dallas, some of the nation's busiest, as blocked roads made it difficult for workers to get to their jobs. A sinkhole also closed a runway at the Dallas/Fort Worth International airport, according to media reports.

About 100,000 customers were without power throughout the state due to high winds and rising waters that caused power poles to snap.

In Houston, Parker said there were about 1,000 vehicles had been submerged in flood waters while in Austin, emergency crews used helicopters and boats to remove people from rushing water.