|Texans wade through flood waters in Beaumont Place|
Around 30,000 people could be forced into shelters by severe rainfall dumped on the state by Tropical Storm Harvey, with weather alerts also issued in Louisiana and Mississippi.
Half a million people are expected to need emergency help, with the worst of the weather yet to recede.
On the eve of his planned trip to the disaster zone with First Lady Melania Trump on Tuesday, the US President said: "It's the biggest ever, they are saying it is the biggest, it's historic.
"It is a historic amount of water, there's never been anything like it."
The storm is set to bring more downpours to Houston, the state's biggest city, with the life-threatening flooding predicted to peak on Wednesday and Thursday.
Up to 20 more inches of rain could fall in the coming days, on top of the 30 inches some areas have experienced so far.
More than 75,000 emergency calls have been processed by Houston's services since Harvey first hit the city.
At least 2,000 people have been rescued so far as a result of the storm, with 12,000 troops deployed to assist with search and rescue efforts.
Mr Trump said he would speak to the US Congress about boosting emergency funds.
"We're dealing with Congress. As you know it's going to be a very expensive situation," he said.
"We want to take care of the people of Texas and Louisiana when that happens - we'll see what happens with the flow, the flow seems to be heading towards Louisiana right now, as we speak - we'll be there for the people of Louisiana also."
|Trump acknowledges scale of Texas flooding|
Texas governor Greg Abbott described Harvey as "if not the largest, one of the largest disasters America has ever faced".
He said the state was "just beginning the process of responding to this storm", with authorities prioritising "protecting and preserving life and rescuing every person we can find".
State officials have said at least eight people appear to have died as a result of Harvey.
Mr Abbott added his second goal was to ensure access to necessities such as food, water and power, with electricity in the process of being restored in areas that most desperately need it.
He said there was a "need to recognise...a new and different normal for this entire region" in the wake of the storm's devastation.
Eighteen of Texas's 254 counties have qualified for federal help due to the disaster, triggering aid from Washington's Federal Emergency Management Agency (FEMA).
The White House insisted FEMA's $3bn balance would be enough to handle the immediate response to the storm, with the agency now prioritising its response to Harvey over earlier disasters in order to prevent it running out of money.
|Houston: A city left submerged under water|
"You're going to have what you need and it's going to go fast," he said.
When US Congress returns next week, it is likely to vote on a multibillion-dollar aid package.
A spokeswoman for House of Representatives speaker Paul Ryan said: "We will help those affected by this terrible disaster.
"The first step in that process is a formal request for resources from the administration."
Mexico, which Mr Trump has continued to insist will pay for a border wall "one way or the other", has offered assistance with the disaster.