CHARLOTTE AMALIE, U.S. Virgin Islands – Omar strengthened into a hurricane and took aim at the Virgin Islands on Wednesday after drenching the southeastern Caribbean.

Authorities issued a hurricane warning for the U.S. Virgin islands as well as Puerto Rico's Vieques and Culebra islands. Hurricane warnings were also in place for St. Martin, the British Virgin Islands, St. Kitts and Nevis and other islands.

In St. Croix, the Hovensa LLC oil refinery, the second-largest in the Western Hemisphere, was shutting down until after the storm passes, said spokesman Alex Moorehead.

Authorities in Puerto Rico prepared shelters in Vieques and Culebra and planned to evacuate elderly residents from the two small islands off the east coast of the main island.

Officials in Puerto Rico, already soaked from several days of rain, warned residents to prepare for a lot more, and medical authorities appealed for blood donations for possible casualties.

Omar was a Category 1 hurricane with winds near 85 mph (140 kph), and the U.S. National Hurricane Center in Miami projected it would strengthen further. The center said the storm would likely pass by Puerto Rico overnight Wednesday, and possibly deliver a direct blow to the U.S. and British Virgin Islands.

In the U.S. Virgin Islands, residents scrambled to stock up on batteries, water and canned goods.

Emergency management director Mark Walters urged islanders to take the warnings seriously.

“This is the time to take those precautions, in terms of getting your family and your personal selves ready for the storm,” he said.

Classes and ferry services were canceled in Puerto Rico and the U.S. Virgin Islands.

The hurricane center said Omar was expected to plow over the northeastern Caribbean islands then head into the central North Atlantic, well away from the U.S. mainland.

Omar's center was located about 235 miles (380 kilometers) south-southwest of St. Croix and 235 (380 kilometers) miles south-southwest of San Juan, Puerto Rico, at 11 a.m. (1500GMT) Wednesday and was moving northeast near 9 mph (15 kph).

Meanwhile, another tropical depression was hugging the coast of Honduras, and a tropical storm warning was in effect for the area.