This satellite image taken Saturday shows clouds in the western Caribbean Sea associated with tropical storm Richard as it moves westward along the coast of Honduras. ((Weather Underground/Associated Press))

Tropical storm Richard gained strength Saturday, lashing the Caribbean coast of Honduras with strong winds and heavy rain that threatened potentially deadly flash floods and landslides.

The storm was expected to pass "near or over" the Honduras coast late Saturday and become a hurricane on Sunday, according to the U.S. National Hurricane Center in Miami. Honduran authorities declared a red, or maximum, alert in four coastal provinces.

Richard is also likely to pass near the Honduran island of Roatan, which is popular with tourists and divers, before making landfall in Belize late Sunday or early Monday.

Hurricane warnings were issued for all of those areas, and storm warnings were also in place for Mexico's southern Caribbean coast.

By Saturday night, Richard was just off the Honduras coast, about 240 kilometres east of the city of La Ceiba, and was moving west at about 11 kilometres per hour.

Its maximum sustained winds strengthened to 110 km/h and were forecast to reach hurricane strength by early Sunday, according to forecasters.

Honduran officials said rain was falling on the eastern province of Gracias a Dios, where floods have been severe in the past.

"Richard is travelling slowly parallel to Honduras' Atlantic coast and causing rains since Friday night on the Caribbean seaboard," said Daniel Posas of the Honduras National Meteorological Service.