A US aircraft carrier entered a zone near the Strait of Hormuz being used by the Iranian navy for wargames, an Iranian official said Thursday amid rising tensions over the key oil-transit channel.

"A US aircraft carrier was spotted inside the manoeuvre zone... by a navy reconnaissance aircraft," Commodore Mahmoud Mousavi, the spokesman for the Iranian exercises, told the official IRNA news agency.

Iranian planes and vessels took video and photos of the US ship and the weaponry and aircraft it was carrying, he added, according to a report carried by state television.

"We are prepared, in accordance with international law, to confront offenders who do not respect our security perimeters during the manoeuvres," the IRIB network quoted Mousavi as saying.

"We suggest that trans-regional forces completely and seriously take any warning issued by any unit of (Iran's) naval forces," he said.

The US aircraft carrier was believed to the USS John C. Stennis, one of the US navy's biggest warships.

US officials announced Wednesday that the ship and its accompanying carrier strike group moved through the Strait of Hormuz, a narrow stretch at the entrance to the Gulf that is the world's most important choke point for oil shipments.

After warnings from the Iranian government and navy this week that Iran could close the strait if threatened by further Western sanctions, the US Defence Department warned Wednesday that such actions "will not be tolerated."

The United States maintains a navy presence in the Gulf in large part to ensure oil traffic there is unhindered.

Iran, which is already subject to several rounds of sanctions over its nuclear programme, has repeatedly said it could target the Strait of Hormuz if attacked or its economy is strangled.

Such a move could cause havoc in the world oil market, disrupting the fragile global economy.

The Islamic republic is halfway through 10 days of navy exercises in international waters to the east of the strait that have included the laying of mines and the use of aerial drones, according to Iranian media.

Missiles and torpedoes were to be test fired in coming days.

The wargames zone covers an area of 2,000 square kilometres (770 square miles) in the Gulf of Oman into the Gulf of Aden, according to Iranian media.

So far, Iran and the United States have limited themselves to rhetoric and naval manoeuvres. But analysts and the oil market are watching the situation carefully, fearing a spark that could ignite open confrontation between the longtime foes.

The United States had proposed a military hotline between Tehran and Washington to defuse any "miscalculations" that could occur as their navies brush against each other. But Iran in September rejected that offer.

Could Iran shut down the Strait of Hormuz?
Washington (AFP) Dec 28, 2011 - Iran could make good on its threat and temporarily shut down or disrupt oil shipping through the strategic Strait of Hormuz, but it would trigger a devastating US military reaction and leave Tehran utterly isolated on the world stage, experts say.

While Iran is invoking a possible closure of the vital strait as it faces the prospect of punitive sanctions, analysts say Tehran may be more inclined to take smaller-scale action short of mining the channel.

With an estimated 2,000 mines in its arsenal, Iran could possibly plant several hundred in the strait before being detected.

Such a move would either shut down the strait or hamper shipping traffic in the narrow channel enough to cause havoc in the world oil market, analysts said.

But laying mines in the strait would represent a clear act of war and Iran would face a massive US military response while antagonizing governments around the world, said Anthony Cordesman of the Center for Strategic and International Studies.

In such a scenario, "everyone in the Gulf is going to support outside intervention," Cordesman told AFP.

Iran would be putting much of its military power and its economy at risk by plunging into a direct confrontation, he said. "This is a country that does not have a modern air force or modern navy."

US-led military action could include "attacks on nuclear facilities, or military production facilities or much broader attacks on Iranian air power," he said.

If the Iranians dropped mines into the Strait of Hormuz, the United States would face the painstaking task of removing the mines or at least clearing out a safe shipping lane while facing hostile fire from Iranian anti-ship missiles and swarms of small boats.

Sophisticated US warplanes likely would be able to take out much of the Iranian missile arsenal and air defenses, but the conflict could drag on depending on Iran's ability to hide its weaponry in an initial assault.

Some optimistic assessments predict the strait would only be closed for a matter of days but other experts take a more pessimistic view.

A 2008 study concluded that "the experience of past mine-warfare campaigns suggests that it could take many weeks, even months, to restore the full flow of commerce, and more time still for the oil markets to be convinced that stability had returned," wrote academic Caitlin Talmadge in International Security, a Harvard University journal.

With Iran relying on the strait as an economic lifeline for its own oil exports, analysts and officials say Tehran might instead opt for lower-level action that would cause a spike in oil prices.

"You have an extremely wide range of asymmetric options that don't have to be anywhere near the strait," Cordesman said.

Iran could choose to harass or board commercial ships in the Gulf and stage searches, said Alireza Nader, an Iranian specialist at the RAND Corporation think tank.

In 2007, Iran seized a crew of British sailors and marines, alleging their boats had crossed into Iranian waters.

The last time Iran confronted American warships in the Gulf, during the Iran-Iraq conflict in the 1980s, it was badly outgunned.

Since then, Iran has sought to bolster its naval power and coastal defenses, investing in more powerful mines, mobile anti-ship missiles, submarines and a large fleet of smaller boats.

A US military officer on Wednesday dismissed Iran's threats over the strait, saying the statements out of Tehran were "mainly rhetoric" and that it was unlikely the Islamic republic would be willing to take such a volatile step.

"Our priority is freedom of navigation. And we would do whatever is necessary to make sure the area is open," said the officer, who spoke on condition of anonymity.

But there would be limited objectives for any US operation it would be tailored "to things that they're doing" near the strait, he said.

Iran is threatening to choke off the Strait of Hormuz as part of a high-stakes game in which it hopes to discourage countries from backing tough sanctions over its nuclear program, said Nader.

There was a growing danger that a low-level provocation by Iran, such as the seizure of a ship belonging to a Gulf state, could trigger an escalating chain reaction -- even though neither Washington or Tehran wants a conflagration, he said.

"The issue is tensions are so high, even if Iran is bluffing, there's a lot of room for miscalculation by both sides," he said.

"Iran is using this as a deterrent but if it goes too far, it might have a war on its hands."