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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