Norway’s radiation authority said Tuesday that Moscow informed it there had been a gas explosion onboard a Russian sub on which a fire killed 14 crew, a claim Moscow strongly denied.

Moscow has revealed little about the incident or the vessel, claiming it was deep-water research submersible, but Russian media reports have said it was a secretive nuclear-powered mini-submarine.

“There has been a gas explosion, confirmed by the Russian authorities,” Per Strand, director of the Norwegian Radiation Protection Authority (NRPA), told AFP.

“We are waiting for information from the Russian side about whether there was a reactor onboard the submarine,” he told AFP, adding that NRPA had not noticed an increase in radiation levels.

Russia’s defense ministry quickly denied the claim.

“There were no notifications sent to the Norwegian side regarding the Russian science research deep water apparatus,” the ministry said in a statement.

The ministry earlier said that 14 Russian seamen were killed on Monday by inhaling poisonous fumes after a fire broke out on a submersible studying seafloor terrain in the far north.

However, the Novaya Gazeta newspaper cited sources as saying that the accident took place on a nuclear mini-submarine AS-12, also known as Losharik, which is capable of going to extreme depths.

Little is known about the AS-12, which was launched in 2003 and is designated for research, rescue and special military operations.

Of the 14 victims, seven were the highest rank given out in the Russian navy, suggesting that it was not an ordinary assignment. It is unknown how many were on board the sub.

The fire was put out and the vessel is now at a military base in the northern city of Severomorsk, which is located on the Kola Peninsula above the Arctic Circle.

President Vladimir Putin ordered a full investigation to find what caused the “tragedy”.