Athens and Ankara -- both NATO members -- are at loggerheads over energy exploration in disputed waters, and alliance commanders are worried about the risk of accidental naval encounters.

NATO has set up a "deconfliction mechanism" to allow the rival militaries to head off trouble, but Stoltenberg said he was also encouraging Turkey's President Recep Tayyip Erdogan and Greece's Prime Minister Kyriakos Mitsotakis to talk.

"This is for us a technical military issue, where we need to make sure that we have the tools to prevent incidents and accidents," Stoltenberg said, after day one of a two-day meeting of defence ministers.

"And it is extremely important to understand that the responsibility for preventing an accident at sea cannot solely be put on the shoulders of the captains and the crew and the soldiers and the pilots who are operating in the eastern Mediterranean.

"And that's reason why I also clearly called on all of us to make sure to make the necessary political decisions to prevent any incidents or accidents."

On Wednesday, Mitsotakis accused Erdogan's Turkey of harbouring "imperialist fantasies", while the Turkish foreign ministry accused Athens and its allies of "maximalist and hostile policies."

In defiance of warnings from Western countries, Turkey has extended a gas exploration mission by its Oruc Reis research vessel in the eastern Mediterranean until October 27.

The Oruc Reis, escorted by military ships, has become the symbol of Ankara's appetite for natural gas in the eastern Mediterranean, where recent discoveries have triggered a rush for the resource.

Athens says Ankara is breaking international law by prospecting in Greek waters, including near the Greek island of Kastellorizo.