DUBAI -- Iran launched a submarine and a destroyer into the Persian Gulf from Bandar Abbas port on Tuesday at the same time as U.S. and allied forces held massive naval exercises in the same waters to practice keeping oil shipping lanes open.

Tehran has repeatedly threatened to close the Strait of Hormuz, a route for oil exports from the gulf, if Iranian nuclear sites are attacked by Israel, which believes Tehran is trying to develop an atomic bomb.

The United States, Britain, France and a number of Middle Eastern states are conducting a naval exercise in the gulf this week, focusing on how to clear mines that Tehran or guerilla groups might deploy to disrupt tanker traffic.

The exercises, with 25 countries participating, are the largest ever of its kind in the region, according to Britain’s Telegraph newspaper 

The multinational force includes three U.S. Nimitz class carrier groups, each of which has more aircraft than Iran’s entire air force, the newspaper said.

The force is also supported by at least 12 battleships, including ballistic missile cruisers, frigates, destroyers and assault ships, which carry thousands of U.S. Marines and special forces, the Telegraph reported.

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Iran's refitted Tareq-901 submarine and Sahand destroyer were launched on the direct orders of Supreme Leader Ayatollah Ali Khamenei, the official IRNA news agency reported.

Iran's 'strong fence'
On the other side of the country, Khamenei visited the northern coastal city of Nowshahr on Tuesday to watch naval cadets practice planting mines, freeing hijacked ships, destroying enemy vessels and jumping from helicopters, his official website said.

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"The armed forces must reach capabilities such that no one can attack the strong fence of the country and the dear people of Iran," Khamenei told army commanders, according to the Iranian Students News Agency.

Iran's Tareq-class submarines are diesel-electric boats that were originally built in Russia in the early 1990s, according to the Nuclear Threat Initiative, a non-profit organization that focuses on security affairs.

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Israeli Prime Minister Benjamin Netanyahu said Sunday that Tehran was close to being able to build a nuclear bomb, fuelling speculation about an Israeli strike. Iran insists its nuclear program is peaceful.

Publicly, Iranian military officials have sounded relaxed about the U.S. naval exercise.

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"This exercise is a defensive exercise and we don't perceive any threats from it," Mohammad Ali Jafari, commander of the Islamic Revolutionary Guard Corps, told local media.

"We are not conducting exercises in response," he said.

NBC News staff and Reuters contributed to this report.

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Updated at 1:37 p.m. ET:

Reuters reports White House officials said Iran's threat to take action if a U.S. aircraft carrier moves into the Gulf showed Tehran was increasingly isolated internationally, faced economic problems from to sanctions and wants to divert attention from its deepening problems.

"It reflects the fact that Iran is in a position of weakness," White House spokesman Jay Carney told reporters Tuesday.

Earlier:

Iran will take action if a U.S. aircraft carrier which left the area because of Iranian naval exercises returns to the Gulf, the state news agency quoted army chief Ataollah Salehi as saying on Tuesday.

"Iran will not repeat its warning ... the enemy's carrier has been moved to the Sea of Oman because of our drill. I recommend and emphasize to the American carrier not to return to the Persian Gulf," Salehi told IRNA.

"I advise, recommend and warn them (the Americans) over the return of this carrier to the Persian Gulf because we are not in the habit of warning more than once," the semi-official Fars news agency quoted Salehi as saying.

Salehi did not name the aircraft carrier or give details of the action Iran might take if it returned. However, last week a spokeswoman for the U.S. 5th Fleet said the USS John C. Stennis had left the Gulf.

Iran completed 10 days of naval exercises in the Gulf on Monday, and said during the drills that if foreign powers imposed sanctions on its crude exports it could shut the Strait of Hormuz, through which 40 percent of the world's traded oil is shipped.

The U.S. Fifth Fleet, which is based in Bahrain, said it would not allow shipping to be disrupted in the strait.

Iran fires missiles
Iran said on Monday it had successfully test-fired two long-range missiles during its naval drill, flexing its military muscle in the face of mounting Western pressure over its controversial nuclear program.

Iran also said it had no intention of closing the Strait of Hormuz but had carried out "mock" exercises on shutting the strategic waterway.

Tehran denies Western accusations that it is secretly trying to build atomic bombs, saying it needs nuclear technology to generate electricity.

The United States and Israel have not ruled out military action against Iran if diplomacy fails to resolve the Islamic state's nuclear row with the West.

The European Union is considering following the United States in banning imports of Iranian crude oil. U.S. President Barack Obama signed new sanctions against Iran into law on Saturday, stepping up the pressure by adding sanctions on financial institutions that deal with Iran's central bank.

Meanwhile, Iran said the new record low of the national currency to the U.S. dollar was not linked to the latest sanctions from the United States targeting the country's central bank.

Foreign Ministry spokesman Ramin Mehmanparast on Tuesday insisted there "is no relation" between the two. He said the American sanctions "have yet to be put into practice."

The Iranian currency's exchange rate hovered late Monday around 18,000 riyals to the dollar, marking a roughly 12 percent slide compared to Sunday's rate of 15,900 riyals to the dollar.

President Barack Obama on Saturday signed into law a bill targeting Iran's central bank as part of the West's efforts to pressure Tehran over its nuclear program. It goes into effect in six months.

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Reuters and The Associated Press contributed to this report.