Ship operations in the US Gulf Coast are facing a major slowdown over the next two days as Hurricane Harvey is expected to strengthen to a major storm.

Houston Pilots have suspended boardings as of Thursday afternoon. The US Coast Guard captain of the port of Houston is still allowing ship transits in the region. But they are monitoring the situation.

"Harvey is expected to become a major hurricane by Friday before it reaches the middle Texas coast," the US National Hurricane Center said in its latest update.

A hurricane watch is in effect for an area covering the port of Corpus Christi on up to the southern end of Galveston Bay.

The are some expectations that the storm could reach Category 4 strength.

"The storm is large, and various impacts to energy infrastructure are going to be significant," said Seaport Global Securities analyst Richard Hastings.

At Thursday noon, Harvey was packing winds of 65 miles per hour (104 kilometres) per hour and headed into a north-northwest direction from the US Gulf.

NWS warns that sea swells will build throughout the day, eventually reaching as high as 20 feet in offshore areas.