Armenia's President Armen Sarkisian used a visit to NATO headquarters on Wednesday to accuse alliance member Turkey of intervening in the conflict over Nagorno-Karabakh.
NATO secretary general Jens Stoltenberg told his guest that NATO, a partner of both Armenia and Azerbaijan, was neutral in the conflict and urged a ceasefire.
But Sarkisian complained that Turkey's backing for Azerbaijan had undermined international efforts to mediate peace.
"It looks like sometimes it's no longer the conflict between Republic of Artsakh, the people of Nagorno-Karabakh, and Azerbaijan, but there is a third party there as well," he said.
Sarkisian accused Ankara of supporting Azerbaijan "politically, diplomatically, heavily militarily" and repeated a long-standing charge that it had facilitated the deployment of Syrian fighters.
"And unfortunately, that third party is a member of NATO and I being here, I could not speak about that third party -- that third party's Turkey," he said, standing by Stoltenberg.
"So I do believe that if a NATO member country like Turkey will stop being a part of the conflict and will contribute to the ceasefire, and to the understanding that there should be a peaceful solution that we will reach a ceasefire."
Stoltenberg did not address the allegations against Turkey at his appearance with the Armenian leader, and instead urged the two main combatant parties to show restraint.
"Both Armenia and Azerbaijan have been valued NATO partners for more than 25 years. NATO is deeply concerned by ongoing violations of the ceasefire which have caused tragic loss of life," he said.
"It is vital that all sides now show restraint, observe the ceasefire and de-escalate. Any targeting of civilians is unacceptable and must stop all must now work towards a sustainable political solution."
Earlier, Armenia's Prime Minister Nikol Pashinyan had ruled out a diplomatic solution to the conflict with Azerbaijan and urged Armenians to volunteer for military service.
Armenia and Azerbaijan have been engaged in fierce fighting for almost a month over Karabakh, a region of Azerbaijan controlled by Armenian separatists.