In the wake of Iran's refusal to rein in its contentious nuclear program at talks in Moscow, the U.S. military buildup in the Persian Gulf is gathering pace, with four more Navy mine countermeasures ships arriving at the weekend.

The Avenger class vessels out of San Diego will join eight U.S. and British navy mine hunters deployed in the region.

Their main mission will be to keep open the strategic Strait of Hormuz, the only way in and out of the gulf, if the Iranians carry through on a threat to close the vital energy artery through which one-fifth of the world's oil supplies pass every day.

Iran is reported to have in excess of 2,000 sea mines, including advanced Russian weapons, that could be used to block the narrow, 112-mile strait.

These could be used in conjunction with Chinese-designed anti-ship missiles and swarm attacks by fleets of armed speedboats manned by naval forces of the elite Revolutionary Guards Corps.

The additional MCM vessels -- the USS Sentry, Devastator, Warrior and Pioneer -- will be based at Manama, capital of the island kingdom of Bahrain and headquarters of the U.S. 5th Fleet.

These slow-moving 1,379-ton ships, all transported to the gulf aboard heavy-lift vessels, will join their forward-deployed sister ships Scout, Gladiator, Ardent and Dextrous, and the British MCM contingent.

The Navy has two aircraft carrier battle groups, headed by the USS Enterprise and the USS Abraham Lincoln, deployed in the gulf and the Arabian Sea. These can put up a powerful force of more than 100 F/A-18 Hornet and Super Hornet strike jets, backed by early warning radar and surveillance aircraft.

The French navy's carrier Charles de Gaulle, carrying multirole Dassault Rafale jets, is also in the region.

Meantime, the USS Ponce, an amphibious transport ship now configured to operate as a mother ship for U.S. Special Forces using helicopters and small patrol boats, was reported transiting the Suez Canal en route to Manama.

It's expected to arrive in Bahrain early in July, which could be a critical period in the long-simmering confrontation between the United States, and its gulf allies led by Saudi Arabia, and Iran.

The European Union has said it plans to initiate an oil boycott against Iran July 1, a move that's expected to tighten the noose around the Islamic Republic's vital oil exports. Those have slumped because of sanctions imposed by the United States that aim to block the financial deals that set up the oil exports.

As the sanctions bite ever deeper into Iran's shaky economy, the clerical regime in Tehran can be expected to seek to retaliate.

Amid expectations that tensions will rise in the region, which contains around 43 percent of the world's oil and vast amounts of its natural gas as well, the Americans appear to be bracing for trouble.

The Navy is reported to have ordered 200 more Tomahawk cruise missiles for its surface ships and nuclear submarines. The Navy has more than 3,000 Tomahawks, which can be used against strategic land targets in Iran, deployed on warships or in storage.

Navy warships and submarines have fired more than 2,000 Tomahawks in combat, from the 1990-91 Gulf War through the Libyan war in 2011.

The current variant, the 1.2-ton RGM-109E Block 4 land attack missile deployed in the gulf, has a range of around 1,000 miles. It can fly at an altitude of 50-100 feet, making it difficult to detect, and has been upgraded to hit moving targets, particularly other ships.

In recent weeks, the U.S. Air Force has deployed an unspecified number of state-of-the-art Lockheed Martin F-22 Raptor fighters, the most advanced operational fighter in the world.

These are believed to be deployed at Al-Dhafra airbase near Abu Dhabi, capital of the United Arab Emirates, a longtime Arab ally.

It has built an aerial strike force in recent years that is aimed at conducting offensive operations against Iran if hostilities break out.

The Air Force has also deployed 20 upgraded Boeing F-15C Eagles of the 104th Fighter Wing of the Massachusetts Air National Guard in the region.

Its location hasn't been announced, but it's probably al-Dhafra or the big U.S. airbase at Al-Udeid in the emirate of Qatar, which houses the U.S. Central Command's forward headquarters.