The U.S. National Hurricane Center in Miami says tropical storm Alex, shown in an image from the National Oceanic and Atmospheric Association, was upgraded to a hurricane Tuesday night. (NOAA/Associated Press)

Tropical storm Alex strengthened to a hurricane Tuesday night as it plowed across the southern Gulf of Mexico heading toward the Mexico-Texas border, forecasters say.

The Category 1 storm had winds of roughly 120 km/h and was expected to make landfall as early as Wednesday night, according to the U.S. National Hurricane Centre. Alex is the first June Atlantic hurricane since 1995.

A hurricane warning was posted for the Texas coast, from Baffin Bay south to the mouth of the Rio Grande river and south to La Cruz, Mexico.

Workers along the south Texas coast were clearing drainage ditches, filling sandbags and positioning heavy equipment and water pumps as well as preparing emergency shelters. Some cities also handed out sandbags to residents and urged people to make preparations.

Mexico's northern Gulf coast braced for heavy rains like those that fell on southern areas and parts of Central America earlier.

"It is a fact we are going to get very heavy rains," said Gov. Fidel Herrera of the Gulf coast state of Veracruz.

Alex, the first named storm of the Atlantic hurricane season, caused flooding and mudslides that led to at least four deaths in Central America over the weekend.

Storm's centre expected to miss Gulf leak

Forecasters said Monday they didn't expect the centre of Alex to pass near the site of the BP oil spill.

A street damaged by tropical storm Alex is seen near Acelhuate River in San Salvador, El Salvador, on Monday. (Luis Galdamez/Reuters)

But the storm could create rough conditions in the Gulf, which would limit the operations of skimming vessels and floating oil-containment booms.

Pulling boats and crews off the water could cost precious time, said Nancy Kinner, co-director of the Coastal Response Research Center at the University of New Hampshire. Equipment has to be stripped down, packed and protected from the force of the storm, and then has to be reassembled and deployed again, she said.

The storm, however, could help break up some of the oil lingering in the Gulf, officials said.

U.S. Coast Guard Admiral Thad Allen, the federal government's point man for the spill response, said the storm wasn't expected to affect two relief wells being drilled, considered the best hope of stopping the leak.

With files from The Associated Press