12:42

Tom Dart is reporting from Houston, where there are fears of disastrous sustained flooding in the country’s fourth-largest city.

More than 300,000 people across Texas were without electricity early on Saturday as Hurricane Harvey trundled inland and threatened to stall, setting up for several days of heavy rainfall that could tally 40 inches by Wednesday in some spots.

Dozens of Houston-area roads were reported flooded on Saturday. At 10am, Houston’s airports announced 380 flight cancellations at George Bush Intercontinental and 114 at Hobby, though a break in bad weather allowed departures to resume at Bush.

Brock Long, the recently appointed administrator of the Federal Emergency Management Agency (Fema), said on Twitter that the storm was transitioning into a “deadly inland event”.

Brock Long (@FEMA_Brock) Citizens of TX, this is now turning into a deadly inland event. Thoughts and prayers are with you. https://t.co/2qqTnl1Anj

In a Saturday morning update, the National Hurricane Center said that though winds had slowed to a maximum of 80mph, Harvey was “moving slowly over Texas producing torrential rains … catastrophic flooding expected over the next few days”.

Houston, about 200 miles northeast of where Harvey made landfall, began seeing wind and rain from the storm on Friday. It is notoriously flood-prone and more than 6.5 million people live in its metropolitan area, though officials decided against ordering a mass evacuation. Levels in the city’s bayous were on the increase, giving rise to the prospect that they would burst their banks and water would inundate surrounding streets if the rain continued as predicted.

Alyssa Pone (@AlyssaPone) Happening now in #Houston: some roads flooded - this is near busy 69 freeway #hurricaneHarvey pic.twitter.com/KWh09LUuez

Traffic was light in Houston on Saturday and many stores were closed, though an exception was a doughnut shop in the suburb of Katy, where Don Mach and his Keeshond dog, Bo, were having breakfast. Mach said he was “very concerned” about Harvey. “We got five-and-a-half inches of rain last night. That came down probably in about four hours,” the 70-year-old said. “That water can only go so many places.”

Oil companies began shutting down operations in and along the Gulf in anticipation of the storm, and gas prices rose.

Economic impact aside, there is anxiety that an unfavourable storm track could result in an environmental tragedy should Harvey provoke flooding that impacts the region’s vast refining and petrochemical facilities and unleashes toxic discharges that spill into adjacent communities or Galveston Bay.

Juan Parras, an environmental campaigner in east Houston, said he was worried that severe flooding or a storm surge could cause leaks or dislodge chemical tanks. “When they move off their concrete base all that oil, whatever’s in those tanks, just goes out into the community and we have a lot of tanks here. We have almost a 52-mile stretch of nothing but refineries and oil tanks,” he said.

The neighbourhoods closest to the plants are some of the least-affluent and most-polluted in the region. “The worst off will be hit the hardest,” Parras said.

Numbers of injuries and fatalities in the wider area were not clear on Saturday.

Corpus Christi police said that road debris and downed power lines were widespread and that an alleged intruder was taken to hospital after being shot by a homeowner. Hundreds of people headed to shelters set up away from the coast. The city of Victoria, 30 miles inland, was also badly hit.

Harvey is the first major natural disaster of Donald Trump’s administration. Trump’s proposed federal budget calls for cuts of $667m to Fema’s funding, but the president was eager to give the impression that he was ready for the challenge. He issued a slew of tweets about the storm. “Closely monitoring #HurricaneHarvey from Camp David. We are leaving nothing to chance. City, State and Federal Govs. working great together!” he wrote on Saturday. A day earlier Trump said he signed a disaster declaration to speed Texas’ access to federal help.

Coastal areas remained vulnerable to storm surge on Saturday while the storm risked spawning isolated tornadoes. One was reported to have struck the Houston suburb of Missouri City early on Saturday, ripping the roofs off dozens of homes.

In Louisiana, preparations were underway in New Orleans as a precaution. Despite billions of dollars spent on rebuilding and protecting the city since it was devastated by Hurricane Katrina in 2005, heavy rain earlier this month caused flooding that exposed problems with its drainage system.