Tankers transporting LNG into Milford Haven are being searched for terrorist mines, sources have revealed.
British media reported that Special Forces frogmen are checking the large vessels before they arrive in UK waters, amid fears that Al Qaeda or Islamic State explosives could be attached.
Special Boat Service and Royal Navy are reportedly searching ships which ferry gas from the Middle East to the UK, to fight an ISIS plot to attach limpet mines to their hulls.
The covert mission is said to have been ongoing for the past two years, following intelligence that terrorists had acquired these types of explosives, which could be detonated once the tankers reach the UK ports and cause potential death and environmental disaster.
A senior Naval source was quoted as stating that there was also a possibility that terrorists could plant mines on ships once they have entered UK waters and cause a major incident.
Limpet mines, which are small and relatively cheap, can be attached to steel, fibre-glass or wooden hulls using magnets, suckers or silent-firing nail guns.
Specialist frogmen can swim to depths of more than 200ft and are trained to defuse limpet mines and IEDs in the dark.