As floodwaters rise in Texas in the wake of Tropical Storm Harvey, there are unconfirmed reports of bodies in submerged cars, and the number of counties declaring a federal disaster continues to climb.
The National Hurricane Centre warned more than 120cm of rain could accumulate in parts of the US state's southeast this week.
Up to 2000 people have been rescued from vehicles and homes as Tropical Storm Harvey batters Texas. Floodwaters were rising in the state capital, Houston, and there were reports of deaths in vehicles but investigations into those reports continued, Chief Darryl Coleman at the Harris County Sheriff's Office said.
The storm has been linked to two confirmed deaths so far.
The National Weather Service said the intense rainfall and flooding were unprecedented and "beyond anything experienced". The National Hurricane Center warned against any travel.
"Catastrophic and life-threatening flooding is occurring in southeastern Texas, and flash flood emergencies are in effect for portions of this area."
The storm is drifting to the southwest at 4km/h, dumping intense rainfall - about 40-60cm - during the week to Friday over the upper Texas coast and southwestern Louisiana.
Isolated rainfall accumulations in the Houston area could reach more than 120cm, the National Hurricane Centre's advisory said.
Storm surges were also expected.
Trying to make our way back into central Houston safely on dry roads. It's very difficult. Every turn we make, another block is flooded. pic.twitter.com/BknN7siTuS— James Cook (@BBCJamesCook) August 27, 2017
Houston mayor Sylvester Turner said 40 boats would be added for rescuing people from high waters that have filled neighbourhoods and highways. "Don't get on the road. Don't assume this storm is over," Mr Turner said.
Harvey came ashore late Friday as the most powerful hurricane to hit Texas in more than 50 years. The death toll is expected to rise as the storm lashes the state for days, triggering more floods, tidal surges and tornadoes.
The centre of the storm is about 200km southwest of Houston, the fourth most populous city in the United States, and is expected to arc slowly toward Houston, bringing heavy rains, until Wednesday.
Like Houston, Galveston was also hit by "epic catastrophic flooding" overnight, the National Weather Service said.
One fatality in Houston has been confirmed - a woman who died as she drove through flooded streets.
On Friday night, a man died in a house fire in the town of Rockport, 48km north of the city of Corpus Christi. Another dozen people in the area suffered injuries including broken bones, another official said.
Harris County sheriff Ed Gonzalez said his deputies had also responded to unconfirmed reports of a deceased woman and child inside a submerged vehicle on a highway near Houston. Another resident told a local TV station of seeing a woman's body floating in the streets during a flash flood in west Houston.
Emergency services told people to climb onto the roofs of their houses rather than into their attics to escape rapidly rising waters. Authorities warned the city's more than two million residents not to leave flooded homes because many of the city's roads were underwater.
Houston police officials said officers were evacuating two flooded apartment complexes.
Rescue efforts are being hampered by strong winds and thousands are without power. The US Coast Guard in Houston has requested more helicopters as more people ask for help.
Governor Greg Abbott has mobilised 1800 military personnel to help with rescue and repair operation
The coastal town of Rockport took a direct hit from the storm, as it made landfall as a category 4 hurricane, leaving streets flooded and strewn with power lines and debris on Saturday.
The storm ripped off roofs, destroyed buildings, flooded coastal towns and had cut off power to nearly 230,000 people in Texas as of Saturday night.
- Reuters / BBC