ISIS is eager to build a terrorist navy to attack cruise ships in the Mediterranean, a senior NATO naval officer says.

Vice-Admiral Clive Johnstone told the U.K. Telegraph the rise of the terror group in Libya offers a chilling hint of its nefarious plans.

Winning ground in that lawless country means access to the sea, and maritime weaponry, according to Johnstone.

"We know they have had ambitions to go off shore, we know they would like to have a maritime arm."

An ISIS navy would also likely threaten important shipping lanes through the Mediterranean.

An attack, he said, might not even be planned, but rather come by way of accidental opportunity when ships simply cross paths.

"I think it won't be a planned, horrible mischievous act, I think it will be an act which is almost a mistake, or it will be an act of random terrorism that will suddenly have extraordinary implications for the western world," he told the British newspaper.

Johnstone said some NATO members are considering redeploying submarines from the Mediterranean to the North Atlantic to counter growing Russian influence there.

That could open more opportunity for an ISIS sea attack.