Syrian leader Bashar al-Assad accused Turkey's President Tayyip Erdogan of being the main instigator in the deadliest fighting between Armenian and Azeri forces for more than 25 years.
In an interview that is likely to exacerbate international frictions over the clashes in the South Caucasus, which entered a 10th day on Tuesday, President Assad also said militants from Syria were being deployed to the conflict area.
Turkey has denied involvement in the fighting in and around Nagorno-Karabakh, a mountain enclave that belongs to Azerbaijan under international law but is governed by ethnic Armenians, and has dismissed accusations that it sent mercenaries to the area.
But Assad told Russian news agency RIA: "He (Erdogan) ... was the main instigator and the initiator of the recent conflict in Nagorno-Karabakh between Azerbaijan and Armenia."
Reiterating accusations first leveled by French President Emmanuel Macron that Turkey has sent Syrian jihadists to fight in the conflict, Assad said: "Damascus can confirm this."
Assad offered no evidence for his allegation. Ankara, which backs rebels trying to oust him, did not respond immediately but has described similar accusations as part of attempts by Armenia to create "dark propaganda" about Turkey.
Almost 300 people have been reported killed in the fighting in Nagorno-Karabakh that erupted on Sept. 27, increasing concern that a wider conflict could be triggered. Turkey has strongly backed Azerbaijan, and Russia has a defense pact with Armenia.
The head of Russia's SVR Foreign Intelligence Service, Sergei Naryshkin, underlined Moscow's concerns by warning that Nagorno-Karabakh could become a launch pad for "international terrorist organizations" to enter Russia and other countries.
Naryshkin said a new war in the region was unacceptable to Moscow and that the fighting differed from previous clashes in the long-running conflict because of the much larger scale and Turkey's strong support for Azerbaijan.
The Kremlin later issued a new appeal for all sides in the conflict to halt fighting immediately.
In the latest fighting, Armenia said Azerbaijan launched a large-scale attack with tanks and artillery on a southern part of the contact line that divides ethnic Armenian and Azeri forces.
Nagorno-Karabakh said four cluster bombs had exploded in the center of Stepanakert, its main administrative center, but gave no further details.
The conflict over the tiny enclave is closely watched abroad, partly because of Nagorno-Karabakh's proximity to pipelines that carry Azeri gas and oil to Europe.
Azerbaijan says Azeri cities outside Nagorno-Karabakh have been struck, and both sides say the other has hit civilian areas. Each denies targeting civilians.
Nagorno-Karabakh said 244 of its servicemen and 19 civilians had been killed since Sept. 27 and many more have been wounded.
The Azeri prosecutor's office said 27 Azeri civilians had been killed in the renewed fighting. Azerbaijan has not disclosed information about its military casualties.
Ceasefire appeals led by the United States, Russia and France, which have for years led mediation efforts, have failed to halt the fighting. Canada and Britain issued another appeal for a ceasefire in London on Tuesday.
Turkish Foreign Minister Mevlut Cavusoglu said during a visit to Azerbaijan's capital, Baku, on Tuesday that international peace efforts had achieved no concrete results in decades and a ceasefire alone would not end the fighting.
"The whole world now needs to understand this cannot go on like this," Cavusoglu said.
Armenia has said it will engage with Washington, Paris and Moscow on peace moves. Azerbaijan says Armenia must set a timetable to withdraw from Nagorno-Karabakh and surrounding Azeri territories, and wants Turkey involved in peace efforts.
Cavusoglu said Turkey wanted "a more active role" and hoped to work with Russia on resolving the conflict. He urged Moscow to step up its peace efforts.