As Haiti prepared to evacuate tens of thousands ahead of the storm, naval commanders ordered the USS Iwo Jima to steam toward Haiti from Suriname.

The amphibious ship has eight CH-46 Sea Knight helicopters and two MH-60 night Hawk choppers and a crew of 1,600, including medical and engineering teams ready to assist in relief efforts after the storm strikes, the US Navy said in a statement.

The military often deploys the versatile amphibious ships for humanitarian missions, as aid supplies and emergency medical teams can be ferried quickly to shore without need of a local airport.

Forecasters said Tomas was expected to regain strength and reach Haiti possibly by Friday.

The storm's violent winds, heavy rain and landslides pose a particular risk to the up to 1.3 million people who inhabit the makeshift tent city camps set up after Haiti's devastating earthquake in January.

Global relief agencies stepped up emergency plans to protect up to 500,000 Haitians from a tropical storm forecast to hit the country which is also facing a cholera epidemic.

Haiti has already suffered the double blow this year of a devastating earthquake that levelled parts of the capital, killing 250,000 people, in January, as well as an unprecedented cholera epidemic that has infected more than 4,700 people further north and killed 337 since late October.