(Reuters) - A U.S. navy ship that had been slated for decommissioning has been sent instead to the Gulf to help mine-clearing operations, the U.S. Fifth Fleet based in Bahrain said, the latest move in a gradual U.S. build-up as tensions with Iran smoulder.

A fleet spokesman in Manama said the USS Ponce, described as a "afloat forward staging base" (AFSB), had arrived on Thursday after undergoing refitting for its new mission.

"Ponce´s primary mission is to support mine countermeasures operations and other missions, such as the ability to provide repair service to other deployed units," the spokesman said in a statement. "Additionally, Ponce also has the capability to embark and launch small riverine craft."

Vice Admiral John Miller, commander of regional navy forces, said the Ponce boasted "enhanced capability to conduct maritime security operations, and gives us greater flexibility to support a wide range of contingencies with our regional partners".

Four U.S. minesweepers arrived in the Gulf last month to bolster the Fifth Fleet and ensure the safety of shipping routes in a waterway through which 40 percent of the world´s seaborne oil exports flow.

They arrived amid a flaring war of nerves between the United States and Iran over Tehran´s disputed nuclear energy programme and Iranian threats to block the Strait of Hormuz, the slender oil shipping channel out of the Gulf, in retaliation for a new European Union ban on its oil exports.

The four minesweepers were ticketed for a seven-month deployment in an area of operations that includes the Gulf, Gulf of Oman, Red Sea and parts of the Indian Ocean.

Tensions have simmered in the Gulf with big-power diplomacy to ease the nuclear dispute at an impasse and Israel renewing veiled threats to attack Iranian atomic sites from the air if sanctions and negotiations fail to curb Iran´s nuclear advances.

A string of hawkish Iranian statements - including a renewed threat to close the Strait and destroy U.S. bases in the region "within minutes" of an attack - over the past week helped thrust benchmark Brent crude oil prices above $100 for the first time since June.

Iran has repeatedly warned of reprisals for any Israeli or U.S.-led strike on its nuclear installations, whose activities it says are purely peaceful but the West suspects are geared to developing the means to produce nuclear arms. (© Thomson Reuters 2012. 07/07/12)