Swaths of Mexico and Texas are bracing for downpours and floods as tropical storm Hermine heads towards the northwestern Gulf coast.

Mexican authorities on Monday warned residents to leave their homes for shelters, while officials in Texas handed out sandbags as a precaution.

The storm comes a few short months after Hurricane Alex, which ripped through the region in June, killing at least 12 people.

Hermine made first made landfall on Monday night about 65 kilometres south of Brownsville, Texas.

The storm could come near hurricane strength, the centre warned.

A hurricane watch was issued Monday for a region stretching from Rio San Fernando, Mexico, to Baffin Bay in Texas.

The area is considered one of the most dangerous in Mexico's turf war between rival drug cartels. Two weeks ago, 72 migrants were killed there in what is regarded as the worst drug gang slaying Mexico has seen so far.

Mexican officials instructed people living in coastal areas to move to shelters, but no official evacuation plans have been set in place.

"We urge the general population to be on alert for possible floods and mudslides," Salvador Trevino, director of civil defence of Tamaulipas state, where Matamoros is located, told The Associated Press.

Further north, emergency crews in Texas prepared pumping equipment and doled out sandbags in Cameron County, the county's emergency management co-ordinator told The Associated Press.

John Cavazos said officials are concerned because the ground is already saturated from earlier rainfall. The storm could pour another 30 centimetres on some areas, he said.

"Anyone living in ... an area that's known to flood, they need to take some precautions," Cavazos said.

No evacuations have been ordered in Texas so far.

Hermine's maximum sustained winds strengthened to 110 km/h Monday as the storm moved closer to the coast.

With files from The Associated Press