With missile batteries, fleets of attack boats and stocks of naval mines, Iran can disrupt traffic through the Strait of Hormuz but probably cannot completely shut down the world's most important oil route, military analysts say. The question for Iran's leadership is whether it is worth the heavy price.

Trying to close the strait would bring down a powerful military response on Iran's head from U.S. forces in the Gulf and turn Tehran's few remaining international allies against it.

Such dire threats illustrate Iran's alarm over new sanctions planned by the U.S. that will target oil exports, the most vital source of revenue for its economy. Iran's leaders shrugged off years of past sanctions by the U.S. and United Nations, mocking them as ineffective. But if it cannot sell its oil, its already suffering economy will be sent into a tailspin.

"It would be very, very difficult for Iran even to impede traffic for a significant period of time," said Jonathan Rue, a senior research analyst at the Washington-based Institute for the Study of War. "They don't have the ability to effectively block the strait."

What the Iranians can do, Rue and other analysts say, is harass traffic through the gulf — anything from stopping tankers to outright attacks. The goal would be to panic markets, drive up shipping insurance rates and spark a rise in world oil prices enough to pressure the United States to back down on sanctions.

The strait would seem to be an easy target, a bottleneck only about 50 kilometres across at its narrowest point between Iran and Oman. Tankers carrying one-sixth of the world's oil supply pass through it.

In recent years, Iran has dramatically ramped up its navy, increasing its arsenal of fast-attack ships, anti-ship missiles and mine-laying vessels. Its elite Revolutionary Guards boasts the most powerful naval forces, with approximately 20,000 men, and at least 10 missile boats boasting C-802 missiles with a range of 120 kilometres, according to a recent report by Anthony Cordesman at the Center for Strategic and International Studies.

The navy has three submarines and some

'A full shutdown would really be the worst case for Iran — that's their last bullet.— Olivier Jakob of the Switzerland-based oil monitor Petromatrix

also deployed a heavy array of anti-ship Seersucker missiles with a range of up to 100 kilometres along its coast overlooking the strait, on mobile platforms that make them harder to hit.

The air force has not received the same level of support, Rue said. "They realize their navies are the best options for inflicting casualties" on the U.S. or Arab Gulf nations.

Still, those forces would not likely be enough to outright seal the strait, given the presence of the U.S. 5th Fleet based in the Gulf nation of Bahrain.

Laying minefields in the Hormuz waters would in theory be the most effective action, forcing time-consuming clearing by U.S. forces and their allies before tankers could move through. But particularly strong currents in the strait make such mining difficult. Moreover, the U.S. and its Gulf allies have extensive surveillance in the area, Rue said, and the United Arab Emirates and Saudi Arabia have both extensively increased their anti-mining capabilities.

Iran's anti-ship missile batteries on the coast are another major threat. But while the missile platforms are mobile, the radar facilities that enable them to target shipping largely are not, making them vulnerable to U.S. strikes.

Hormuz is in the territorial waters of Iran and Oman, but it is considered an international strait where free passage is guaranteed, meaning that under international law, closing it by any nation would be considered an act of war. Russia and China, Iran's main allies that have protected it from stronger UN sanctions, would have little choice but to respond.

Hormuz's closure would also be a heavier blow to Iran than any sanctions hitting the approximately 2.5 million barrels a day of oil it exports, which provide some 80 per cent of its revenue. Not only do all of its oil exports go through the strait, but also most of its imports, including vital gasoline supplies.

"A full shutdown would really be the worst case for Iran — that's their last bullet," said Olivier Jakob of the Switzerland-based oil monitor Petromatrix.