A new British Royal Navy nuclear-powered submarine ran aground off the coast of north-west Scotland on Friday, the Ministry of Defence said.
"This is not a nuclear incident," a spokesman said. "There are no injuries to personnel and the submarine is watertight."
The MoD identified the boat as an Astute-class submarine, adding that it was not armed with nuclear warheads. It said it was "assessing the situation."
The recently commissioned HMS Astute is the first of a new class of nuclear-powered submarines.
HMS Astute was commissioned into the navy in August and was undergoing sea trials before becoming operational.
According to the Royal Navy's website it is the largest and most advanced sub the navy operates and will progressively replace the "Swiftsure" and "Trafalgar" classes.
The Maritime and Coastguard said it was alerted to the incident at about 1 a.m. ET and had sent an emergency tug boat to assist.
The coastguard said the submarine appeared to have got into trouble on rocks off the Isle of Skye.
"This is an MoD matter and we have an emergency vessel standing by," a spokesman for the coastguard said.
Witness Ross McKerlich told BBC News: "She is now high and dry and we can see just the tips of her propeller.
"It looks as if it has developed a slight list. There was a helicopter hovering over the top and there are still just the two naval vessels from the local base.
"Earlier in the day, they did have ropes and they were trying to tow (the submarine)."
It's not the first time a British nuclear submarine has run aground off northwest Scotland.
In 2002 HMS Trafalgar got into trouble off the Isle of Skye during a military exercise and two crew were slightly injured in the accident.
As part of a major defence review unveiled on Tuesday, the government confirmed it would go ahead with an order for seven new Astute-class submarines, built by BAE Systems
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