New York, New York - Titanic director James Cameron said many warnings were ignored about the safety of the tourist submersible that imploded near the famous shipwreck, killing five people.

Cameron, who is himself a deep-sea explorer, said the sub had been the source of widespread concern in the close-knit ocean exploration community, and drew parallels to the 1912 ocean liner sinking in which around 1,500 people died.

"I'm struck by the similarity of the Titanic disaster itself, where the captain was repeatedly warned about ice ahead of his ship, and yet he steamed at full speed into an ice field on a moonless night, and many people died as a result," Cameron told ABC News.

"And for a very similar tragedy, where warnings went unheeded, to take place at the same exact site, with all the diving that's going on all around the world, I think it's just astonishing."

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"It's really quite surreal."

The US Coast Guard confirmed Thursday that the small sub, operated by OceanGate Expeditions, had suffered a "catastrophic implosion" in the ocean depths, ending a multinational search-and-rescue operation that captivated the world.

Cameron – who in 2012 became the first person to make a solo dive to the very deepest part of the ocean, in a submersible he designed and had built – said the risk of a sub imploding under pressure was always "first and foremost" in engineers' minds.

"That's the nightmare that we've all lived with" since entering the field of deep exploration, he said, pointing to the sector's very strong safety record over recent decades.

But "many people in the community were very concerned about this sub," he said.

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"A number of the top players in the deep-submergence engineering community even wrote letters to the company, saying that what they were doing was too experimental to carry passengers, and that it needed to be certified."

The Hollywood legend added that he had personally known one of the lost submersible passengers, French ocean explorer Paul-Henri "PH" Nargeolet.

"It's a very small community. I've known PH for 25 years. For him to have died tragically in this way is almost impossible for me to process."

Cameron has visited the Titanic shipwreck many times in the course of – and since – directing his 1997 epic starring Leonardo DiCaprio and Kate Winslet, which won a joint-record 11 Oscars.

"I know the wreck site very well... I actually calculated that I spent more time on the ship than the captain did back in the day," he said.

Cameron has also directed the underwater disaster movie The Abyss and multiple deep-sea documentaries.