An ageing American naval ship converted into a "floating base" has arrived off the coast of Bahrain to support countermine operations in the Gulf, according to the US Navy.

The deployment of the USS Ponce marks the latest in a gradual buildup of American forces in the region since tensions rose with Iran in December over its nuclear program, with Tehran threatening to possibly close the strategic Strait of Hormuz.

Equipped with a helicopter landing deck and space for troops, the amphibious transport ship, in service since 1971, can support an array of missions as a "forward staging base" involving Special Forces, countermine operations or repair work for warships, officials said on Friday.

The Ponce “provides us with an enhanced capability to conduct maritime security operations, and gives us greater flexibility to support a wide range of contingencies with our regional partners,” said Vice Admiral John Miller, Commander of Naval Forces in the Middle East.

The ship, which has a crew of 150 civilians and 55 sailors, departed Norfolk, Virginia, on June 1 and arrived in Bahrain last Thursday, officials said.

Meanwhile, Iran has prepared a plan for closing the Strait of Hormuz but will only use it if threatened, Iran's non-official Mehr News Agency reported Saturday.

Mehr quoted Chief-of-Staff of Iran's Armed Forces Major General Seyed Hassan Firouzabadi as saying that the Islamic Republic would act rationally when deciding whether to close the strategically important waterway through which an estimated 20% of the world's oil supply passes.

Iran would only close the strait if its interests were “in serious trouble.” Iran has been feeling the heat of increased economic sanctions recently, with new restrictions imposed last week by the European Union on the sale of Iranian oil.