Secretary General Jens Stoltenberg's comments after talks with Turkish Foreign Minister Mevlut Cavusoglu came as fighting between Azerbaijani and Armenian separatist forces entered its second week with nearly 250 people killed.

"We are deeply concerned by the escalation of hostilities. All sides should immediately cease fighting and find a way forward towards a peaceful resolution," Stoltenberg said at a joint press appearance with Cavusoglu.

"And I expect Turkey to use its considerable influence to calm tensions."

Turkish President Recep Tayyip Erdogan has urged fellow Muslim Azerbaijan to press on with its campaign until it reclaims lands it lost in an early 1990s war that claimed 30,000 lives as the Soviet Union fell apart.

Cavusoglu said NATO should approach the escalation "in a balanced fashion".

"Everyone, particularly NATO, should call on Armenia to withdraw in order for there to be a solution to this problem within the frame of international law," Cavusoglu said.

Nagorno-Karabakh is viewed as part of Azerbaijan by the United Nations and was never recognised as an independent state by Armenia.

But Yerevan fully supports the region and has historically hostile relations with Azerbaijan.

- Eastern Mediterranean dispute -

Stoltenberg's visit to Turkey came during a new spell of tensions with one of its most important member states.

Turkey contributes one of the largest forces to the Western military alliances and plays a crucial role in Libya and the Middle East.

But Turkey's hunt for natural gas in the eastern Mediterranean has resulted in its staging rival war games with fellow NATO member Greece.

The conflict began to ease when the two NATO neighbours agreed last month to resume direct negotiations for the first time since 2016. No date for the Istanbul talks has been announced.

Turkey also pulled back a drilling ship from contested waters around Cyprus after the European Union on Friday threatened to sanction Ankara.

The European Union said the Yavuz vessel's return to a Turkish port on Monday "constitutes another welcome step towards de-escalation in the eastern Mediterranean".

Stoltenberg meanwhile welcomed an agreement by Athens and Ankara last week to set up a military hotline to head off accidental clashes.

"I welcome this and pay tribute to both allies for their efforts and we stand ready to develop it further," the NATO chief said.

"The de-confliciton mechanism can help create the space for diplomatic efforts."

Stoltenberg is due to meet Erdogan in Ankara later Monday.