Leaders across much of the Caribbean closed schools and government offices on Wednesday as tropical storm Isaac swept toward the region, and the U.S. military postponed hearings for Sept. 11 prisoners ahead of a storm that could sweep across Cuba and perhaps eventually menace Florida as a hurricane.

The storm was 120 kilometres east-northeast of Dominica early Wednesday afternoon, with maximum sustained winds of 75 km/h. Isaac was moving west at 33 km/h and is expected to become a hurricane by Thursday, according to the U.S. National Hurricane Center.

Dominica Prime Minister Roosevelt Skerrit urged people to stay home from work on Wednesday.

"I want us all to be safe," he said. "I don't want lives to be lost."

Military authorities in Guantanamo Bay, Cuba, cancelled several days of pretrial hearings in the case of five prisoners charged in the Sept. 11 attacks. They also planned to evacuate about 200 people, including legal teams and relatives of Sept. 11 victims.

Isaac also posed a possible threat to Florida during next week's Republican National Convention in Tampa, according to forecasters, though the hurricane's track was uncertain.

In Puerto Rico, Gov. Luis Fortuno declared a state of emergency and activated the National Guard. He also cancelled classes and closed government agencies on Wednesday. Federal officials also closed the popular San Felipe del Morro castle in Old San Juan. The storm was expected to pass just south of Puerto Rico on Thursday.

The storm's centre was expected to move over the Leeward Islands on Wednesday evening, and forecasters said it is expected to hit the Dominican Republic as a hurricane early Friday. It is then expected to hit Cuba as a tropical storm.