- Video report by ITV News Washington Correspondent Robert Moore
Donald Trump has pledged "every asset at his command" to help those hit by Storm Harvey as it emerged a family of six died trying to escape the Texas floodwater.
The two grandparents and four children, aged six to 16, died after their van sank into a bayou in Houston, a relative said.
The children's father, Sam Saldivar, escaped the vehicle and tried to rescue the others but could only watch it sink into the water.
At least nine lives have been lost to the hurricane-turned-tropical storm, including a woman in Houston when a tree fell onto her trailer home.
The US president, who has flown to visit the rescuers battling the rising floodwater, said "there's probably never been anything like" the storm that has wreaked havoc and warned "recovery will be a long and difficult road".
Harvey has generated an amount of rain that would normally be seen only once in more than 1,000 years and forced thousands to flee their flooded homes.
Despite being downgraded from a hurricane to a tropical storm, after four days of rain Harvey continues to cause chaos with waters reaching neck height in some places.
Mr Trump, who has declared a state of emergency in Texas and neighbouring Louisiana, said protecting lives was his top priority.
The president and First Lady Melania Trump will visit the Texas cities of Corpus Christi and Austin on Tuesday to see the relief efforts at first hand.
"Every asset at my command is at the disposal of local officials," Mr Trump said as he dealt with the first major natural disaster of his presidency.
He promised a rapid federal response for those affected, saying: "You're going to have what you need and it's going to be fast."
Mr Trump added: "We are one American family - we hurt together, we struggle together and, believe me, we endure together.
"To the people of Texas and Louisiana, we are 100% with you."
More than $1 million (£772,000) has been raised in just over a day after a social media appeal by NFL star JJ Watt of the Houston Texans.
He hailed the "phenomenal" support for the storm victims and said he would ensure the donations would go "directly to the people who need it".
With nearly two more feet of rain expected, authorities are concerned the worst is yet to come with Louisiana bracing for the worst.
Rescuers are continuing to pluck people from the floodwaters - with at least 2,000 helped so far, according to Houston Police Chief Art Acevedo.
Up to 20 more inches of rain could fall in the coming days, on top of the more than 30 inches some places have already seen.
Harvey was the fiercest hurricane to hit the US in 13 years and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.