Hurricane Omar was churning toward the Virgin islands Wednesday gaining strength as authorities issued warnings across much of the northeastern Caribbean.

With its center 380 km (235 miles) southwest of St. Croix and about 375 km (235 miles) south-southwest of San Juan, Puerto Rico, the US National Hurricane Center warned at 1500 GMT: "additional strengthening is forecast during the next 24 hours and Omar should be a Category 2 hurricane by the time it reaches the northern Leeward islands."

The hurricane -- packing maximum sustained winds of near 140 kilometers (85 miles) per hour -- was moving slowly northeast toward San Juan.

A hurricane warning was issued for the US Virgin Islands and islands of Vieques and Culebra, outlying isles of Puerto Rico.

Authorities also issues a hurricane warning, which means hurricane conditions are expected within 24 hours, for St. Martin -- the Caribbean's main air hub after Puerto Rico -- Saba, St. Eustatius, St. Barthelemy, the British Virgin Islands, Anguilla and St. Kitts a Nevis.

"Preparations to protect life and property should be rushed to completion," the NHC said.

The storm was churning on its forward track about 15 km/hr (nine mph) "and this motion is expected to continue with a gradual increase in forward speed over the next day or two.

"On this track Omar is forecast to move through the northern Leeward islands (just east of Puerto Rico) late (Wednesday) and early Thursday," the NHC added.

"Omar is expected to produce total rainfall amounts of (10-20 cm) four to eight inches over portions of the Netherlands Antilles with maximum amounts of (30 cm) 12 inches possible. Rainfall amounts of (five-10 cm) two to four inches with maximum amounts of (15 cm) six inches are possible across extreme northwestern and north-central Venezuela and the northern Guajira peninsula. Total rainfall amounts of five-10 inches with maximum amounts up to 20 inches will be possible across Puerto Rico and the northern Leeward islands. These rains could produce life-threatening flash floods and mud slides," the NHC warned.

The busy 2008 hurricane season has included devastating Hurricanes Gustav and Ike, which caused millions of dollars in damage in Haiti, Cuba and the United States.

Hurricanes and tropical storms have killed hundreds across the Caribbean and Mexico, with Haiti the worst hit.