The United States Navy has released a warning to shipping in the Middle East against possible attacks by the extremist Islamic group al Qaeda on oil tankers in the Gulf.
The Navy's Maritime Liaison Office in Bahrain warned ships to "exercise extreme caution" in the Gulf region, which pumps a quarter of the world's oil.
The advisory comes on the eve of the anniversary marking the September 11 attacks on the United States, but gave no specific details on the timing or means of attack.
"According to unconfirmed reports circulating within the regional shipping community, the al Qaeda terrorist group has planned attacks against oil tankers transiting the Arabian Gulf and Horn of Africa areas," the statement said.
The warning pushed world oil prices sharply higher. Brent crude jumped 51 cents to a year-high of $29 a barrel. U.S. crude hit a 19-month high above $30.
A spokesman for the U.S. Navy in Bahrain said it was the first time the Navy had released such a notice following the September 11 attacks by suspected al Qaeda members.
About 25 percent of the world's oil supply comes from the Gulf region, most of it shipped in tankers through the narrow Strait of Hormuz between Oman and Iran.
"Shipmasters should exercise extreme caution when transiting strategic chokepoints such as the Strait of Hormuz or Bab-el-Mandeb, or sailing in traditional high-threat areas such as along the Horn of Africa and other confined waters," the Navy statement said.
"Coalition forces are alert to the potential threat and are currently on patrol in the Arabian Gulf, Red Sea and Gulf of Oman."
Two years ago, 17 U.S. servicemen were killed when an explosion ripped through U.S. destroyer Cole, docked in Yemen, just south of the Gulf. Washington blamed that bombing, like the September 11 attack, on Osama Bin Laden.