The United States and about 20 other nations will hold a major anti-mine operation near the Gulf in September, the Pentagon said Tuesday, after Iran threatened it could block oil shipments.

The maneuvers, including a symposium on mine countermeasures, will be held between September 16 to 27, Pentagon spokesman George Little said.

"It's a defensive exercise aimed at preserving freedom of navigation in international waterways in the Middle East," Little said.

The United States has deployed the USS Ponce to serve as logistical backup in the anti-mine effort.

It has doubled to eight the number of minesweepers in the Gulf and sent in four MH-53 Sea Stallion anti-mine choppers as well as underwater drones.

"This is not an exercise that's aimed to deliver a message to Iran," Little said. "This is an exercise that's designed to, within this multinational forum, increase our capabilities and cooperation."

Washington has warned Tehran not to block the strategic Strait of Hormuz, which the Islamic republic has threatened to do if international sanctions against its nuclear program begin to bite.

The Pentagon is also building a missile defense radar station at a covert location in Qatar, The Wall Street Journal reported Tuesday.

The site will be part of a system intended to defend the interests of the United States and its regional allies against Iranian rockets, unnamed US officials told the newspaper.

A similar radar has existed on Mount Keren in the Negev Desert since 2008 and another is installed in Turkey as part of NATO's missile defense shield.