Tropical storm Ophelia Wednesday started gaining strength northeast off the South American coastline and headed towards the Caribbean, the U.S. National Hurricane Center (NHC) said.

Mexico's National Meteorological Service (SMN) warned people across the southern states of Oaxaca, Chiapas as well as on the Yucatan peninsula and in Tabasco state on the Gulf coast should be alert to the heavy rains expected later after Tropical storm Hilary formed off the Pacific coast of Chiapas.

By noon local time, Ophelia was moving in westward direction over the Atlantic with maximum sustained winds up to 95 km per hour from 75 km per hour in the morning. It is expected to reach the outlying Caribbean islands of Barbados and Dominica by Saturday before approaching Puerto Rico late Sunday, said the NHC.

"Some gradual strengthening is possible during the next day or so," said the Miami-based NHC in its latest report, adding Ophelia is moving westwards at 20 km per hour and expected to increase its speed when it gets closer to the land.

The NHC still forecasts at least three to five major storms or hurricanes to be formed in the next two months before the end of November. A tropical storm becomes a Category 1 hurricane once sustained maximum winds reach 119 km per hour.

Even though Ophelia hasn't caused any danger right now, people across Central America, Mexico and the Caribbean remain alert to the prospects of more rains as the region already has received much heavier rainfall than normal at this time of the year.

Soils in highland areas in the regions have already been saturated after several months of torrential rains from the 2011 hurricane season, making them vulnerable to deadly landslides and flashfloods that have already killed tens of people across Mexico and the Caribbean since early August.

Mexico's SMN urged local authorities along the Pacific coast to be watching closely as mountains and coastal regions in Chiapas are expected more than 50 millimeters of rainfall after the passing of Hilary.

The SMN said it had issued warnings for the Pacific coast area of Chiapas State, the Soconusco mountain highlands in Chiapas as well as southern Oaxaca because of the heavy rains provoked by Hilary, which "has the potential to develop into a major storm."