Water levels from catastrophic flooding that has killed at least nine people in US state might not peak until Wednesday or Thursday

Donald Trump is to visit Texas on Tuesday, where catastrophic floods that have already killed nine people are forecast to worsen over the next 48 hours.

Responding to the first natural disaster to test his presidency, Trump said on Monday that the cost of recovering from tropical storm Harvey would be “very expensive”, but pledged that “the federal government stands ready, willing and able to support that effort”.

The White House said he would visit Corpus Christi and Austin, but not the worst hit area – Houston – over fears that the trip could hamper relief efforts.

Tens of thousands of homes in and around Houston, the nation’s fourth-largest city, are semi-submerged and major roads resemble rivers after an unprecedented 30 inches (75cm) of rain fell on the city since Friday. Overall, the storm has dumped 9tn gallons on the region – as much in two days as it normally receives in a year. Authorities have warned that 30,000 people will be forced to seek shelter.

A further 20 inches of rain is forecast to fall by the end of the week, prompting a warning that flood levels will not peak in south-east Texas until Wednesday and Thursday.

Facebook Twitter Pinterest Evacuees are helped to dry land after their homes were inundated with flooding from Hurricane Harvey. Photograph: Joe Raedle/Getty Images

“We are not out of the woods yet, not by a long shot,” said Elaine Duke, the acting US homeland security secretary.

Concern was also mounting about destructive flooding in neighbouring Louisiana, where the storm is heading. Trump issued a federal state of emergency for the the state. The National Hurricane Center warned of “ongoing catastrophic and life-threatening flooding” spreading from Texas to Louisiana, with total rainfall of 50 inches (1.27 metres) in some parts.

At least nine people were reported to have been killed in the storm, including six members of the same family whose van was swept away in the flood water and a man in his 60s who was presumed drowned after trying to swim to safety. It is feared that the death toll could rise substantially once the arduous clean-up operation gets under way.

Art Acevedo, the Houston police chief, told reporters that 2,000 people had been rescued from flooding in the city, with 185 distress calls still waiting for help. At least 7,000 people were seeking shelter at Houston’s George R Brown convention centre, which has been turned into a makeshift shelter with space for only 5,000 beds.

The authorities have called for a community effort to help deal with the disaster, including a police call for boat owners to help rescue those stranded in their homes.

“There’s water everywhere. Please help. I’m scared,” one resident of downtown Houston, Aisha Nelson, told ABC News from atop her two-storey building.

Houston’s mayor, Sylvester Turner, was facing questions about why he chose not to order an evacuation of the city’s 2.3 million residents in advance of Harvey.

Turner said an evacuation of so many people in so concentrated an urban area would merely have compounded the crisis. “If you think the situation right now is bad, and you give an order to evacuate, you are creating a nightmare,” he said.

Facebook Twitter Pinterest Flooding in La Grange, Texas. Photograph: Cal Sport Media/Rex/Shutterstock

The Brazos river that runs through Fort Bend could reach 59ft by Thursday, 4ft above the previous record set last year, forcing thousands more locals to flee under a mandatory evacuation order. Further misery was added to the picture by the decision to release water from two major reservoirs, Addicks and Barker, into the Buffalo bayou that runs through Houston.

When Hurricane Harvey made landfall late on Friday it was the most powerful hurricane to strike the US in 13 years. It morphed into a tropical storm and lingered over Houston, bombarding the city with vast quantities of rain.

The Houston Chronicle summed up the human tragedy with a list of numbers: 18 US Coast Guard helicopters used for rooftop rescues; 56,000 calls to 911 just in the first 15 hours.

Images on social media conveyed the agony; above all, a photograph of a group of six older people and their cat sitting in the communal room of La Vita Bella assisted-living home in Dickinson, green-grey water lapping at their waists. All the residents were rescued by National Guard troops. On Monday, the Texas governor, Greg Abbott, activated all 12,000 such troops in the state.