Iranian naval forces over the weekend reportedly warned off a foreign helicopter near the area where it is conducting large-scale exercises in the Persian Gulf.

The helicopter's country of origin wasn't disclosed but the semi-official Fars News Agency indicated it belonged to a non-Middle Eastern country.

"The helicopter ignored the first two warnings but left the area after the third and severe warning," the agency quoted Rear Adm. Seyed Mahmoud Mousavi as saying.

The incident is the only one in naval maneuvers of keen interest to Persian Gulf countries allied with the United States as well as the United States itself, which operates its 5th Fleet from Bahrain and helps protect the Strait of Hormuz, a major route for petroleum tankers.

Iran has threatened in the past to close the Strait of Hormuz in any military confrontation with the United States and NATO over nuclear program.

Velayat (Supremacy) 90, as the Iranian maneuvers are called, has been touted by Tehran as its largest naval exercise to date to demonstrate "Iran's military prowess and defense capabilities in the international waters, convey a message of peace and friendship to regional countries and test the newest military equipment."

Adm. Habibollah Sayari, Iran's navy chief, added that Iran's newest anti-ship missiles and torpedoes would be used.

The exercise may indeed be a "message of peace" but it is unmistakably a demonstration of force following new economic sanctions against Iran and the continuing threat of military action by Israel -- alone or in concert with the United States -- over the nuclear program, which is suspected as being designed to build nuclear weapons.

Iran's threats -- implicit as well as explicit -- to close the Strait of Hormuz shouldn't be taken likely if push comes to shove in the nuclear weapons standoff or in other confrontations with Tehran over its attempts to spread its influence in the Middle East, including Iraq.

Iran's naval forces may be dwarfed by overall U.S. naval forces but could still prove a not-insignificant challenge in hostilities.

The Strait of Hormuz, connecting the Persian Gulf with the Gulf of Oman and the Arabian Sea is just 21 miles wide at one point. The width of its shipping lane is only 2 miles, separated by a 2-mile buffer zone. About 13 oil tankers passed through it each day in 2009, carrying petroleum -- principally to Asian markets - with about the same number passing through each day to load crude from Middle East oil terminals.

The U.S. Energy Information Administration says about one-third of sea-born oil shipments passes through the choke point. The sinking of one or two vessels would block the passageway for a considerable length of time and Iran has the capability to do so.

In addition to its three destroyers and five frigates, Iran possesses about 19 submarines -- including mini-subs -- nearly 200 fast-attack boats armed with anti-ship missiles and possessing mine-laying capabilities and additional mine-laying vessels.

Many of its naval bases are located near shipping lanes.

And Iran in the past has demonstrated aggressiveness in the gulf, where Tehran and its neighbors of decades have been in dispute over a number of islands.

That aggressiveness on occasion involved European powers. In 2007, Iranian forces seized 15 British sailors and marines who were interdicting and searching vessels as part of coalition operations in Iraq. Iran claimed they had been in its territorial waters.

Velayat 90, which began Saturday, is to last 10 days over a 1,200-mile area. The show of force underlines Tehran's defiance in the face of Western pressure over its nuclear program.

The Strait of Hormuz: key route for world's oil
Dubai (AFP) Dec 28, 2011 - The Strait of Hormuz, which Iran has threatened to block if the West applies sanctions on its oil exports, is a strategically important waterway through which 40 percent of the world's oil transits.

The strait links the Gulf, bordered by petroleum-rich states like Bahrain, Kuwait, Qatar, Saudi Arabia and the United Arab Emirates, with Oman.

Iran's Vice President Mohammad Reza Rahimi warned Tuesday: "If sanctions are adopted against Iranian oil, not a drop of oil will pass through the Strait of Hormuz."

Several Western states are considering oil sanctions against Iran over its nuclear programme, accusing it of seeking to develop nuclear weapons.

The narrow Strait of Hormuz has a maximum width of some 50 kilometres (30 miles) and a maximum depth of 60 metres (200 feet).

The corridor is dotted with little-inhabited desert islands, which are nevertheless strategically important, notably the Iranian islands of Hormuz and Qeshm and Hengam off Bandar Abbas.

Oman's Musandam Peninsula juts out to the Strait of Hormuz towards Iran, separated from the rest of the sultanate by land belonging to the UAE.

With their back to the UAE coast, the three strategic and disputed islands of Greater Tunb, Lesser Tunb and Abu Musa serve as observation posts on all Gulf States: the UAE, Qatar, Bahrain, Saudi Arabia, Kuwait, Iraq and Iran.

Iran under the Western-backed shah gained control of the islands in 1971, as Britain granted independence to its Gulf protectorates and withdrew its forces.

Abu Musa, the only inhabited island of the three, was placed under joint administration in a deal with Sharjah, now part of the UAE.

But the UAE says the Iranians have since taken control of all access to the strategic island and installed an airport and military base there.

Iran's Revolutionary Guard, the ideological army of the Iranian regime, controls naval operations in the Gulf and the Strait of Hormuz.

In the event of war, the Islamic republic could block access to the oil-rich region, according to a US intelligence report published in late 2009.

More than a third of the world's tanker-borne oil passes through the Strait of Hormuz.

The United States maintains a navy presence in the Gulf in large part to ensure that passage remains free.

Iran is carrying out navy exercises in international waters to the east of the Strait of Hormuz.