Turkey should use its “considerable influence” in the region to calm the renewed conflict in the Armenian breakaway region of Nagorno-Karabakh, Nato head Jens Stoltenberg said on Monday.
Mr Stoltenberg held talks with Turkish Foreign Minister Mevlut Cavusoglu on the nine-day conflict between Azerbaijani and Armenian separatist forces. So far nearly 250 people have been killed in the fighting.
"We are deeply concerned by the escalation of hostilities," Mr Stoltenberg said alongside Mr Cavusoglu in Ankara.
"All sides should immediately cease fighting and find a way forward towards a peaceful resolution.
"And I expect Turkey to use its considerable influence to calm tensions."
Armenia has accused Ankara of sending Syrian mercenaries to fight alongside Azerbaijan, as it did in Libya for the Government of National Accord despite widespread international condemnation.
Turkish President Recep Tayyip Erdogan has urged close ally 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.
Mr Cavusoglu said Nato should approach the escalation "in a balanced fashion".
But Yerevan fully supports the region and has historically hostile relations with Azerbaijan.
Armenia and Azerbaijan on Monday accused each other of attacking civilian areas, where scores of people have been killed in the fiercest clashes since the 1990s.
Mr Stoltenberg called for a ceasefire.
"It is extremely important that we convey a very clear message to all parties that they should cease fighting immediately; that we should support all efforts to find a peaceful, negotiated solution," he said.
Armenian Prime Minister Nikol Pashinyan said that “a ceasefire could be established only if Turkey were removed from the South Caucasus”.
Turkey’s government has denied sending arms or foreign fighters, while publicly siding with Azerbaijan in the dispute.
On Sunday, Russian Foreign Minister Sergey Lavrov called for an immediate ceasefire and said Moscow was ready to help seek a solution to the conflict through the Organisation for Security and Co-operation in Europe.
At the same time the Foreign Ministry of Iran, which has nearly 760 kilometres of border with Azerbaijan and a short frontier with Armenia, on Monday said it was working on a peace plan to end the fighting.
Ministry spokesman Saeed Khatibzadeh did not give details but said it was consulting all related parties.
Mr Khatibzadeh also warned both sides against spilling the hostilities on to Iran’s territory.