Tropical Storm Erika is strengthening as it races across the Gulf of Mexico towards the Texas and Mexico coasts.

The storm is expected to make landfall early Saturday as a small hurricane.

Forecasters say it is heading due west toward the bordering coastal cities of Brownsville, Texas and Matamoros, Mexico.

A hurricane warning has been posted for the central Texas coastline from Port O'Connor, about 160 kilometers south of Houston, to Brownsville. Port O'Connor is where hurricane Claudette crashed ashore last month, killing two people.

The Mexican government has issued a hurricane warning for northeastern Mexico from the Rio Grande River to Boca Santa Maria.

Erika is the fifth tropical storm of the season. A tropical storm becomes a hurricane if its sustained winds reach 120 kilometers per hour.