Hurricane Sally drew closer to the US Gulf Coast on Tuesday morning, bringing heavy rains and surging water ahead of its expected landfall as a Category 2 hurricane, with the chance of further strengthening possible.

The second strong storm in less than a month to threaten the region, Sally's winds increased to 100 miles per hour (155 kph), and late Monday was 90 miles (145 km) east of the mouth of the Mississippi River, the National Hurricane Center (NHC) said.

It could wallop the Mississippi and Alabama coasts on Tuesday with devastating winds of up to 110 mph, on the cusp of becoming a Category 3 hurricane on the five-step Saffir-Simpson scale of intensity, the NHC said.

Hurricanes are considered to have the potential for devastating damage when they have sustained winds past 111 mph (179 kph).

Mississippi and Louisiana called for evacuations of low-lying areas and President Donald Trump issued an emergency disaster declaration for both states. Alabama closed the state's beaches and recommended evacuations of residents in low-lying areas.

Mobile, Alabama Mayor Sandy Stimpson warned residents he expected a "tremendous amount of flooding" and said the city was barricading intersections likely to see high water.

Ports, schools and businesses closed along the coast. The US Coast Guard restricted travel on the lower Mississippi River in New Orleans to the Gulf, and closed the ports of Pascagoula and Gulfport, Mississippi, and Mobile, Alabama.

Energy companies buttoned up or halted oil refineries and pulled workers from offshore oil and gas production platforms.

The hurricane is expected to dump between 8 and 16 inches (20 to 40 cm) of rain on the coast, with isolated 24-inch downpours, and cause widespread river flooding.

Mississippi appears more likely for landfall, but Sally's biggest threat is that it will be a "rainmaker" across a wide swath of the Gulf Coast, with 3 to 4 inches (7.62 to 10.2 cm) in areas as far inland as Atlanta, said Jim Foerster, chief meteorologist at DTN, an energy, agriculture and weather data provider.

Sally is the 18th named storm in the Atlantic this year and will be the eighth of tropical storm or hurricane strength to hit the United States - something "very rare if not a record" said Dan Kottlowski, senior meteorologist at AccuWeather, noting that accurate data on historic tropical storms can be elusive.