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A British nuclear-powered submarine has been towed free after running aground off the coast of northwest Scotland during a routine exchange of crew members.

Tug boats pulled the $1.6bn HMS Astute free on Friday as the tide rose.

Military officials played down the significance of the embarrassing accident involving the navy's latest and biggest submarine, saying there was no chance of any nuclear material leaking from the vessel.

"This is not a nuclear incident," a spokesman at the ministry of defence said. "There are no injuries to personnel and the submarine is watertight."

A navy spokesman described the accident as a "relatively minor incident".

The submarine, which was snagged on rocks off the Isle of Skye, is an Astute-class submarine but was not armed with nuclear warheads.

The Maritime and Coastguard said it was alerted to the incident at about 0720 GMT and had sent an emergency tug boat to assist.

"At some point she touched the rudder on the bottom and they weren't able to get her off immediately," a spokesman said.

Astute warcraft

David Black, a consultant with Jane's Strategic Advisory Services in London, told Al Jazeera the vessel "is one of the new class of nuclear attack submarine for the navy and was launched in 2007".

"It's about 7,200 tonnes and is reportedly one of the most advanced ... submarines currently in service," he said.

It is not the first time a British nuclear submarine has run aground off northwest Scotland, where the surrounding coastline is used as a training ground by the navy.

In 2002 HMS Trafalgar got into trouble off the Isle of Skye during a military exercise and two crew were slightly injured.

As part of a major defence review unveiled on Tuesday, the government confirmed that it would go ahead with an order for seven new Astute-class submarines, built by BAE Systems.

The HMS Astute is still undergoing sea trials before becoming operational.

The design, development and manufacture of the first three Astute submarines cost $6.1bn.

According to the Royal Navy's website, it is the largest and most advanced submarine the navy operates and will progressively replace the "Swiftsure" and "Trafalgar" classes.

Source: Al Jazeera and agencies