US President Donald Trump defended the state and federal response to deadly flash flooding in Texas on Friday as he visited the stricken Hill Country region, where at least 120 people, including dozens of children, perished a week ago.

During a roundtable discussion after touring Kerr County, the epicentre of the disaster, Trump praised both Texas Governor Greg Abbott and Homeland Security Secretary Kristi Noem for their response, saying they both did an "incredible job."

The Trump administration, as well as local and state officials, has faced mounting questions over whether more could have been done to protect and warn residents ahead of the flooding, which struck with astonishing speed in the pre-dawn hours on July 4, the US Independence Day holiday.

Trump reacted with anger when a reporter said some families affected by the floods had expressed frustration that warnings did not go out sooner.

"I think everyone did an incredible job under the circumstances," he said. "I don't know who you are, but only a very evil person would ask a question like that."

Some critics have questioned whether the administration's spending cuts at the National Weather Service and the Federal Emergency Management Agency, which coordinates the US government's disaster response efforts, might have exacerbated the calamity.

Trump officials have said that cuts had no impact on the NWS's ability to forecast the storms, despite some vacancies in local offices.

But the president has largely sidestepped questions about his plans to shrink or abolish FEMA and reassign many of its key functions to state and local governments.

"I'll tell you some other time," Trump said on Tuesday, when asked by a reporter about FEMA.

Before the most recent flooding, Kerr County declined to install an early-warning system after failing to secure state money to cover the cost.

Search teams on Friday were still combing through muddy debris littering parts of the Hill Country in central Texas, looking for the dozens still listed as missing, but no survivors have been found since the day of the floods.

Heavy rains sent a wall of water raging down the Guadalupe River early on July 4, causing the deadliest disaster of the Republican president's nearly six-month term in office. (Reuters)