London (CNN Business) Crude oil futures surged more than 4% on Thursday after two tankers were attacked in the Gulf of Oman, a key waterway used to transport energy supplies from the Middle East.

The Japanese owner of the Kokuka Courageous told CNN the 27,000-ton tanker had been attacked "by some sort of shell" off Fujairah, a port in the United Arab Emirates. The ship suffered damage to its starboard hull.

The ship's operator, Singapore-based BSM Ship Management, said in a statement that the Kokuka Courageous was about 14 nautical miles from the coast of Iran.

A second tanker, the Front Altair, was also attacked while sailing between the UAE and Iran, the Norwegian Maritime Agency said in a statement.

"There were reportedly three detonations on board the ship," the agency said.

Smoke billowing from a tanker said to have been attacked off the coast of Oman. CNN has not independently verified this image.

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