At least 12 people, including five soldiers, have died in Texas due to torrential rain in the past week.
Hundreds of homes have been damaged and three prison facilities have been evacuated.
The US army and local rescue teams were using boats, helicopters and sniffer dogs to search for four soldiers who went missing when their military vehicle overturned in a flood-swollen creek on Thursday at Fort Hood in central Texas.
Five other soldiers in the vehicle were killed while three more who survived, a military official told a news conference.
"This tragedy extends well beyond Fort Hood and the outpouring of support from around the country is sincerely appreciated," Major General John Uberti said.
The vehicle overturned at a low-water crossing and military officials have not said why the convoy was training near a swollen waterway.
The sprawling army post, covering an area about 15 times larger than Manhattan, was closing down flood-hit roads when the accident took place.
There was likely one more flood-related death in the state, San Antonio police said, after recovering the body of a man caught in metal bars at a river drainage site in the city.
The Texas Department of Criminal Justice began evacuating about 1,700 inmates at its Ramsey Unit in Rosharon, about 50km south of Houston, due to flooding along the Brazos River.
It evacuated about 2,600 inmates from two other facilities earlier this week due to flooding on the same river.
Many of the inmates were sent to other units with available beds, it said.
The National Weather Service has placed large parts of Texas and Louisiana on a flash flood watch, including Houston and New Orleans.
Heavy rains are forecast to hit Houston and eastern Texas through the weekend, likely causing more flooding, it said. Some areas could see as much 18cm.
"Due to already saturated soils, even fairly brief, intense rainfall rates can easily cause roadway and low-land flooding over urban areas," it said.
More than 150 flights were cancelled at airports in Dallas and Houston today due to the weather, according to tracking service FlightAware.com.