‍Floodwaters from Tropical Storm Harvey, which has already killed at least seven people in Texas and was expected to drive tens of thousands from their homes, will likely rise in the coming days, officials warned on Monday as heavy rains continued to pound the US Gulf Coast.

National Guard troops, police officers, rescue workers and civilians raced in helicopters, boats and special high-water trucks to rescue the hundreds of people still stranded in and around Houston, the nation's fourth-largest city. 

The storm came ashore on Friday near Corpus Christi - 220 miles (354 km) south of Houston - and became the most powerful hurricane to strike Texas in more than 50 years. However, the worst is far from over, as the National Weather Service have issued numerous flood warnings across the region.

US President Donald Trump plans to go to Texas on Tuesday to survey the damage and may also visit Louisiana where the storm is now dumping rain. Trump, facing the biggest US natural disaster since he took office in January, has signed disaster proclamations for Texas and Louisiana, triggering federal relief efforts.

Harvey has killed at least six people in Harris County, where Houston is located, said Tricia Bentley, a spokeswoman for the county coroner's office, including a man who died in a house fire on Friday night and an elderly woman driving through flooded streets on the city's west side the next day. 

Both of Houston's major airports were shut down, along with most major highways, railways, and hospitals, where patients were evacuated over the weekend. As of Monday evening, 267,000 Texans were left without power in the southeast corner of the state. 

The Harris County Office of Homeland Security and Emergency Management issued a "shelter in place" warning to residents of La Porte and Shoreacres, about 25 miles (40 km) east of Houston after a chemical leak was caused by a ruptured pipeline. 

As stunned families surveyed the wreckage of destroyed homes and roads flooded or clogged with debris, Texas Governor Greg Abbott warned Houstonians to brace for a long recovery, saying "we need to recognize this is going to be a new and different normal for this entire region."

Harvey is expected to linger over Texas' Gulf Coast for the next few days, dropping another 10 to 20 inches (25 to 51 cm) of rain, with threats of flooding extending into Louisiana. 

In scenes evoking the aftermath of Hurricane Katrina in 2005, police and Coast Guard teams have rescued at least 2,000 people so far, plucking many from rooftops by helicopter, as they urged the hundreds more believed to be marooned in flooded houses to hang towels or sheets outside to alert rescuers.

Harvey's center was about 100 miles (160 km) south of Houston and forecast to arc slowly toward the city through Wednesday, with the worst floods expected later that day and continuing into Thursday.

Schools and office buildings were closed throughout the metropolitan area - home to 6.8 million people - as chest-high water filled some neighborhoods in the low-lying city. 

Torrential rain also hit areas more than 150 miles (240 km) away, swelling rivers and causing a surge that moved towards the Houston area, where numerous rivers and streams have already been breached. About 5,500 people were in shelters as of Monday morning, city officials said, with Federal Emergency Management Agency director Brock Long forecasting that 30,000 would eventually be housed temporarily in shelters. Governor Abbott, who had suggested on Friday that people leave the area, declined to second-guess the mayor on Monday, telling reporters, "Decisions about evacuations are something that is behind us."

Gasoline futures hit their highest in two years as Harvey shut down multiple refiners, knocking out about 13 percent of total US refining capacity, based on company reports and Reuters estimates. 

Almost half of the US refining capacity is in the Gulf region. The nation's second-largest refinery was shut down in Baytown, and the nation's largest refinery in Port Arthur was weighing shutting down with a final decision expected Tuesday, sources told Reuters. 

The floods could destroy as much as $20 billion in insured property, making it one of the most expensive storms for US insurers, according to Wall Street analysts.