(Reuters) - Tropical Storm Isaac has nearly become a hurricane, with significant storm surge and flood threat from rainfall expected along the northern Gulf coast, the U.S. National Hurricane Center said on Tuesday.

The storm was located about 105 miles southeast of the mouth of the Mississippi River, with maximum sustained winds of 70 miles per hour (110 kmph), the NHC said.

Isaac was moving toward the northwest at a speed near 7 mph (11 kmph).

Once ashore, the storm could wreak havoc on low-lying fuel refineries along the Gulf Coast that account for about 40 percent of U.S. refining capacity.

(Reporting by NR Sethuraman; Editing by Gerald E. McCormick)