Pirates commandeered a cargo ship in the territorial waters of Oman, dramatically extending their area of operation and threatening for the first time shipping in and out of the oil-rich Persian Gulf.

The hijacking, reported over the weekend, took place Friday. It follows another failed attack nearby earlier last week.

A fleet of international naval vessels assembled to combat a surge in pirate attacks around the Horn of Africa may be driving pirates further afield, navy officials said. Seasonally bad weather may also be pushing pirates northward.

Pirate attacks started soaring last summer in the Gulf of Aden and off the east coast of Africa, boosting insurance and shipping rates in the region. Pirates have attacked oil and petrochemical carriers, including a fully laden Saudi Arabian oil tanker that was released after a ransom payment.

But maritime officials have so far viewed Persian Gulf shipping as relatively secure. Friday's attack is the first reported, successful hijacking so close to the Strait of Hormuz, the mouth to the Persian Gulf. Ships with as much as 17 million barrels of oil, or about 40 percent of all sea-borne oil traded, pass through the strait each day, according to the U.S. Energy Information Administration.

"Pirates continue to expand their area of attack," said Lt. Nathan Christensen, a spokesman for the U.S. Fifth Fleet, based in Bahrain in the Persian Gulf.

Click here to read the full report from WSJ.com