Evacuations are underway in Texas as thousands of people head inland to seek shelter from Hurricane Ike.

The third major storm of the season is due to strike the Texan coast on Friday evening or early on Saturday. Ike was rated as a category two storm today but is predicted to reach category three or even a powerful four on the five-tier Saffir-Simpson scale.

When the storm slams into the mainland this weekend it is forecast that devastating winds could reach up to 131mph. Hurricanes are notoriously difficult to predict however and the authorities in some coastal regions are reluctant to implement early evacuations.

Residents of parts of Galveston and four counties south and east of Houston were issued with mandatory evacuation orders yesterday and hundreds of weak or ill patients were being driven to San Antonio as President Bush declared a state of emergency in Texas.

Houston itself is likely to feel the power of Ike on Saturday, but no immediate evacuations have been ordered in Harris County, which includes America’s fourth largest city.

Jeff Masters, a meteorologist for Weather Underground, said: “I expect a lot of damage in Houston from this storm.”

Mr Masters expects a “huge storm surge” to affect at least 100 miles of the Texan Gulf Coast over the weekend.

Houston officials were expecting flooding. Patrick Trahan, spokesman for the city, said: “Based on the current forecast we would expect to see some flooding based solely on the surge in some low-lying areas.”

About 1 million people live in the coastal counties between Corpus Christi and Galveston. The Galveston-Houston area could be on the edge of hurricane-force wind gusts, even if the storm makes land 100 miles to the southwest as some forecasts say is likely.

Storm surges in the Houston-Galveston area could reach 10 to 14 feet in a Category 3 storm, and as much as 20 feet for a Category 4.

The surge in Galveston Bay could push floodwaters into Houston, damaging areas that include the nation’s biggest oil refinery and NASA’s Johnson Space Centre.

Shell, the Anglo-Dutch oil giant, evacuated employees from its offshore installations yesterday when the hurricane hit the Gulf of Mexico, where many oil refineries are based.

Texas is home to 26 refineries that account for a fourth of US refining capacity, and most are clustered along the Gulf Coast in places such as Houston, Port Arthur and Corpus Christi.

Refineries are built to withstand high winds, but flooding can disrupt operations and power outages can shut down equipment for days or weeks. An extended outage could force oil prices up significantly.

The US has announced it will provide $10 million (£5.7 million) in aid to help Haiti to recover from the recent storms, a spokesman for the White House said after around 100 people died as Ike lashed the Caribbean.

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