Trump arrives in Texas to survey flood damage

US President Donald Trump has arrived in Texas to survey the damage caused by flash flooding and to meet with local officials, first responders and relatives of some of the 120 people killed in the July 4 disaster.

His visit comes at the end of a week of mounting questions about the government response to the deadly deluge and hopes of finding any more survivors nearly extinguished.

Search teams have combed through muddy debris littering parts of the Texas Hill Country, looking for the dozens still listed as missing, but no survivors have been found since the day of flooding.

Last Friday, torrential rains sent a wall of water raging down the Guadalupe River in the pre-dawn hours of the US Independence Day holiday.

The disaster is the deadliest of the Republican president's nearly six-month term in office.

As sun poked through dark clouds on Friday, search crews in hard hats painstakingly walked inch-by-inch along the ruined banks of the river, marking damage and looking through wreckage.

"It's a horrible thing," Trump told reporters as he departed the White House with First Lady Melania Trump.

"Nobody can even believe it, such a thing - that much water that fast."

Air Force One touched down at Kelly Field air base in San Antonio at about noon.

Trump travelled by helicopter to Kerrville, the epicentre of the flooding in south-central Texas, where he spoke with relatives of the victims and emergency responders, according to a White House official.

He was set to listen to a briefing from local officials and tour sites in Kerr County.

The county is located in what is known as "flash flood alley," a region that has been the scene of some of the deadliest floods in the United States.

More than 30cm of rain fell in less than an hour on July 4.

Flood gauges showed the river's height rose from about 30cm to 10.4 metres in a matter of hours, cascading over its banks and sweeping away trees and structures in its path.

Kerr County officials say more than 160 people remain unaccounted for, although experts say that the number of people reported missing after disasters is often inflated.

The dead include at least 36 children, many of whom were campers at the nearly century-old Camp Mystic, an all-girls Christian summer retreat on the banks of the river.

Local and federal officials have faced scrutiny for their response, including questions about whether they could have done more to warn people of the rising floodwaters.

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

In an interview with NBC on Thursday ahead of the trip, Trump appeared to support any fresh initiative to install such alarms.

"After having seen this horrible event, I would imagine you'd put alarms up in some form," Trump said.

The Texas state legislature will convene in a special session later this month to investigate the flooding and provide disaster relief funding.

The US Senate's top Democrat, Chuck Schumer, on Monday asked a government watchdog to investigate whether cuts at the National Weather Service affected the forecasting agency's response.

The NWS has defended its forecasting and emergency management, noting it assigned extra forecasters to two Texas offices over the holiday weekend.

Trump's administration has said the agency was sufficiently staffed and responded adequately.

On NBC, Trump described the flooding as a "once-in-every-200-year event".