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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Firefighters work to extinguish fire at the Roslyakovo shipyard in the northern Russian region of Murmansk.

Updated at 6:45 a.m. ET: Russia said on Friday it had doused a raging blaze aboard a nuclear submarine after nearly a full day and night, by partially submerging the vessel after battling the flames with water from helicopters and tug boats.

There was no radiation leak and crew inside the submarine were monitoring the stricken vessel's nuclear reactors which had been shut down, Russian officials said.

At least nine people were injured fighting the flames which witnesses quoted by local media said rose 30 feet above the Yekaterinburg submarine at the navy ship yard in the Murmansk region of northern Russia.

"The fire on the submarine has been totally extinguished," Emergencies Minister Sergei Shoigu told officials leading the firefighting effort, more than twenty hours after the blaze began Thursday afternoon at 7:20 a.m. ET.

His remarks were reported by Interfax news agency.

Updated 4:53 a.m. ET: An unspecified number of crew remain inside a burning nuclear submarine that caught fire on Thursday at an Arctic shipyard, Russia's military said Friday.

Seven other crew were sent to hospitals after inhaling toxic fumes, the country's defense ministry said.

State-owned news agency RIA reported military proescutor spokesman Alexander Grigoriev saying: "Some of [the crew] are still on the submarine. They consist of those servicemen who are ensuring the safety of of the nuclear submarine."

• STORY: Fire on Russian nuclear submarine

It said there has been no radiation leak from the fire on board the submarine Yekaterinburg, which was in drydock.

Fire brigades were still struggling to put out the blaze on Friday at 12 p.m. local time (4 a.m. ET).

The military said the fire had begun on wooden scaffolding and then engulfed the submarine's rubber-coated outer hull. It said the sub's nuclear reactor had been shut down and its 16 nuclear-tipped missiles had been unloaded before the repairs.

The ministry's statement left it unclear whether the crew members inside the vessel were trapped there or ordered to stay inside.

Emergency workers said efforts to partially sink the submarine at the dock had failed to fully extinguish the fire.

A defense ministry spokesman said on Thursday the nuclear reactor had been shut down and all weapons had been removed from the Yekaterinburg, which launched an intercontinental ballistic missile from the Barents Sea at a firing range thousands of miles away in Kamchatka as recently as July.

The Yekaterinburg is a Delta-IV-class nuclear-powered submarine that normally carries 16 nuclear-tipped intercontinental ballistic missiles. It was built in 1984.

Most modern submarines' outer hulls are covered with rubber to make them less noisy and more difficult for an enemy to detect.

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 The Associated Press and msnbc.com staff contributed to this report.