Hollywood director James Cameron Tuesday mourned a "tragic loss" after a deep sea research vessel imploded nearly 10 kilometres (six miles) beneath the surface of the Pacific Ocean.

The Nereus robotic research vehicle was exploring the Kermadec Trench, several hundred kilometres north of New Zealand, when it was crushed by the extreme water pressure.

"I feel like I've lost a friend," Cameron posted Tuesday on the Facebook page of the US-based research body the Woods Hole Oceanographic Institution (WHOI), which was operating the Nereus.

"Nereus was an amazing, groundbreaking robot and the only currently active vehicle in the world that could reach the extreme depths of the ocean trenches. This is a tragic loss for deep science.

"They've not only lost a child, they've lost a great opportunity to explore one of the ocean's deep trenches -- the last great frontier for exploration on our planet," Cameron wrote.

The "Titanic" and "Avatar" director is also a renowned deep sea explorer. In 2012 he made a record-setting solo dive to the world's deepest ocean point in the Marianas Trench in his Deepsea Challenger submersible, which he later donated to WHOI.

The unmanned Nereus was part of a mission to explore an area of ocean from 6,000 - 11,000 metres (19,685-36,089 feet) deep.

It is likely to have imploded under pressure of up to 16,000 pounds per square inch, WHOI said in a statement.