The accident, which also injured one crewman, occurred while the HMS Tireless, a hunter-killer submarine which does not carry nuclear missiles, was under the ice cap in the Arctic Ocean during a joint exercise Wednesday.

In Washington, US Navy spokeswoman Lieutenant Li Cohen told AFP that a self-contained oxygen-generation candle, part of the air purification system, exploded, causing the accident.

Britain's defence ministry did not immediately comment on the US account, nor confirm there was an explosion, but said there was no broader threat posed by the accident.

"The submarine was never in any danger, its nuclear reactor was unaffected, it quickly surfaced and is completely safe," it said in a statement.

"At this early stage, it is thought that the accident involved a piece of air-purification equipment in the forward section of the submarine.

"The ship's company dealt with the incident quickly and professionally and, as a result, there is only superficial damage to the forward compartment.

"It was submerged under the ice cap at the time of the incident. The crew are trained in surfacing quickly through the ice, and did so in exemplary fashion."

The ministry said relatives of the two submariners killed, who have not been named, have been informed while the injured sailor had been taken to a US military hospital and was expected to make a full recovery. US Navy officers added that the injured sailor was airlifted to a hospital at Elmendorf Air Force Base in Anchorage, Alaska.

The Times newspaper said one was killed in the blast and the other died from inhaling fumes. It quoted Royal Navy sources as saying the smoke had been contained in the area of the explosion and there was no other damage to equipment.

The Royal Navy and US Navy have been conducting joint exercises in the Arctic since 1986.

Air purification equipment is fitted to all Trafalgar Class submarines, of which Tireless is one of seven in the Royal Navy.

Oxygen-generation candles create the gas through a chemical reaction, and some of the candles burn at high temperatures during the reaction.

The defence ministry said the equipment had a 100 percent safety record to date, but as a precaution its use on other submarines has been restricted until safety checks can be carried out.

Britain's former first sea lord -- the country's top navy brass -- retired admiral Sir Alan West told BBC radio the generators had been fitted in 2001 and replaced an older, less safe system.

"They have been in there since 2001 and we have had no problems with them up until now," he said.

On Arctic manoeuvres, submarine commanders know at all times where the thinnest part of the ice is and can head for it in the event of an emergency, he added.

Praising a textbook operation, he said: "The ship's crew performed brilliantly."

HMS Tireless, based at Devonport, in Plymouth, southwest England, was launched in 1984.

The vessel has had a controversial history, having to dock in the British overseas territory of Gibraltar in May 2000 after a coolant leak was discovered in its nuclear reactor. It remained there for nearly a year, sparking tens of thousands of people to protest in neighbouring Spain.

An defence ministry spokesman said the coolant leak in 2000-2001 was entirely unrelated to Wednesday's incident, and described the 2001 update to the air-purification machinery as "routine".

The dangers faced by submariners were highlighted in August 2000, when the Russian Kursk sank in the Barents Sea, killing 118 crew.

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