The head of NATO said Monday he expected Turkey -- a key ally of Azerbaijan -- to use its "considerable" influence to calm the conflict in the ethnic Armenian separatist region of Nagorno-Karabakh.

Secretary General Jens Stoltenberg's comments in Ankara came as fighting between Azerbaijani and Armenian separatist forces entered its second week with at least 260 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 after talks with Turkish Foreign Minister Mevlut 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 takes back lands it lost in a war in the early 1990s that killed 30,000 as Nagorno-Karabakh broke away.

He said moments before starting his own talks with the NATO commander that Azerbaijan was "responding to an attack and saving Karabakh from its occupation".

"We, Turkey, say that we are always on the Azerbaijan side," Erdogan said in a televised address.

"As long as the Karabakh issue is not resolved, it will not be possible to end the unrest and conflict in the region."

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.

An Azeri protester waves Azerbaijani and Turkish national flags in a demonstration in Istanbul on Sunday
An Azeri protester waves Azerbaijani and Turkish national flags in a demonstration in Istanbul on Sunday AFP / Ozan KOSE

Stoltenberg's visit to Turkey came during a new spell of tensions with its strategically vital member state.

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

But Turkey's hunt for natural gas deposits in disputed eastern Mediterranean waters sparked a regional crisis in August that forced fellow NATO member Greece to stage war games with its top European allies in a show of force.

Those tensions began to ease when the two 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".

The Turkish energy ministry said the ship was undergoing maintenance and refuelling in preparation for "drilling activities in a new location".

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

"The de-confliction mechanism can help create the space for diplomatic efforts," the NATO chief said.

Stoltenberg next travels to Athens on Tuesday for talks with Greek Prime Minister Kyriakos Mitsotakis.