Five people have been killed and nearly 1,200 rescued by emergency workers after Houston was suddenly inundated with deadly floodwaters amid a torrential rainstorm.

"There's flooding in every part of Houston," said Mayor Sylvester Turner at a press conference. He assured stranded citizens: "We will rescue you."

Residents were trapped in sinking cars and stranded in attics as floodwaters rose higher and higher in their homes. Waters climbed with lightning speed as the city was pelted with 17 inches (43cm) of rain in 24 hours causing creeks and seven bayous to slip their shores, flooding areas in the Texas city seldom swamped ever before.

Governor Greg Abbott has already declared a state of emergency in nine counties, making the storm the biggest flood event in Texas since Tropical Storm Allison in 2001, ABC News reported. Allison killed 41 people and caused $5bn (£3.5bn) in property damage in a single county.

City officials pleaded with residents to stay put. "This is a life-threatening emergency," the city said on an emergency website. "Houston residents should avoid travel at all costs."

Most if not all of the victims killed in the flood so far were apparently drowned when they became trapped in their submerged vehicles, including the driver of an 18-wheeler found dead in a high-water area, CNN reported.

There have been 44,000 power outages reported, and all schools have been closed. Nine hospitals in the region were closed to additional patients because of the flooding. And yet more torrential rains are expected.

In one dramatic near-death situation, a man was filmed driving into floodwaters by a journalist crew reporting on the emergency. As the car began to float reporter Steve Campion of local ABC13 shouted: "Dude, you've got to get out of the car." When the driver poked his head out and asked: "What should I do?" Campion responded: "Swim! Swim!"

The driver managed to squeeze from his window and swim toward Campion, who helped pull him to safety. He apparently had no idea how deep the water was when he drove into it. By the time the two looked back, the car was completely submerged.