HOUSTON - Torrential rains have killed at least 15 people in Texas and Oklahoma, including three in Houston where floods turned streets into rivers and led to about 1,000 calls for help in the fourth-most populous US city, officials said on Tuesday.The death toll is set to rise with numerous people still missing in Texas after the storms slammed the states during the Memorial Day weekend, causing record floods that destroyed hundreds of homes, swept away bridges, and even unearthed a coffin from a Houston cemetery. It washed ashore on the banks of a bayou."A lot of folks drove their car into high water and had to abandon those vehicles," Mayor Annise Parker said at a news conference.Two of the dead in Houston were found in their cars and another was spotted floating in a bayou. The Oklahoma Medical Examiner's office said six people have died in weather-related incidents over the holiday weekend in the state.Though Parker said parts of the city were unscathed, more than 1,000 vehicles were submerged in the Houston floods and people took instead to bicycles, kayaks and surfboards to navigate water-covered streets. The Houston Fire Department brought about 500 people to safety in boats, local media reports said.US President Barack Obama said on Tuesday 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 at least 40 Texas counties, including Harris County, home to Houston.