The National Guard said Tuesday it is preparing to deploy 20,000 to 30,000 of its troops to aid in the response and recovery after historic Texas flooding caused by Hurricane Harvey.

The troops have not yet been requested by Gov. Greg Abbott, but the Guard began preparing for the massive deployment due to the extent of the disaster and its anticipation of a prolonged recovery effort, Maj. Gen James Witham, the National Guard Bureau director of domestic operations, told reporters at the Pentagon.

"I would like to emphasize that our response to this hurricane has been different to anything we've experienced before and we expect it to be much longer in terms of the response phase in terms of what we could normally see with a hurricane," Witham said.

Texas has so far called up 3,000 of its state's National Guard troops and that number was expected to increase to 4,000 over the coming day, he said. Overall, the state could choose to deploy a total of about 12,000 state Guardsmen who are available.

Harvey blew over southeastern Texas on Friday and has dumped rain for days, flooding the Houston metro area, sending tens of thousands of residents into shelters, and forcing thousands of rescues by helicopter and boat.

Witham said it was "debatable" whether local authorities in Texas had been too slow to request assistance by the National Guard as the scope of the disaster was becoming more apparent.

"In many cases, the request for assistance not only for the National Guard but federal forces may not have been anticipated quick enough but we are providing everything we can as quickly as the state asks for it," Witham said.

The chief of the National Guard Bureau, Gen. Joseph Lengyel, used his own authority to prepare for the potential massive deployment of troops over time.