Sensitive to the threat of wider confrontation, Russia and Turkey are for now limiting involvement in the Nagorno-Karabakh conflict to providing humanitarian assistance and some military aid.
Ankara sees its strong backing for Azerbaijan over the fighting with ethnic Armenians in the mountain enclave as part of efforts to boost Turkey's international clout. Moscow is determined to defend its own interests in the South Caucasus.
But neither wants to be sucked into an all-out war, and private military contractors say Moscow and Ankara are largely turning a blind eye to the role of mercenaries - possibly fighting on both sides - to avoid stoking tensions.
NATO member Turkey, which has stepped up arms supplies to Azerbaijan in recent years, is likely to refrain from deeper military involvement if its ally continues to advance in Nagorno-Karabakh, military and political analysts said.
Russia, which has a defence pact with Armenia, also has good relations with Azerbaijan and is unlikely to become directly involved militarily unless Azerbaijan launches a deliberate attack on Armenia, they said.
"The fundamental question is: does the Kremlin want the return of Nagorno-Karabakh to Azerbaijan?" said Pierre Razoux, academic director at France's Mediterranean Foundation of Strategic Studies.
Thousands are feared killed since fighting flared on September 27 in the breakaway territory, which is internationally recognised as part of Azerbaijan but populated and controlled by ethnic Armenians.
Azeri President Ilham Aliyev said on Sunday his country's forces had taken Shusha, the second-largest city in Nagorno-Karabakh.
Armenian officials denied this but Turkish President Tayyip Erdogan congratulated his "Azeri brothers" and said he believed Aliyev's statement was "a sign" that Azerbaijan would soon regain control of more territory.
Turkey's support for Azerbaijan has been vital, and Azerbaijan's superior weaponry and battlefield advances have reduced its incentive to reach a lasting peace deal.
Ankara denies its troops are involved in fighting but Aliyev has acknowledged some Turkish F-16 fighter jets remained in Azerbaijan after a military drill this summer, and there are reports of Russian and Turkish drones being used by both sides.
Russia is Armenia's main arms supplier though it also sells weapons to Azerbaijan which, like Armenia, was for decades part of the Soviet Union.
"All Assistance Required"
Russia has said it will give "all assistance required" should the conflict spill onto "the territory of Armenia".
A Russian private military contractor, speaking to Reuters on condition of anonymity, cited unconfirmed information from a colleague that Russian mercenaries had gone to Nagorno-Karabakh.
Alexander Borodai, a former leader of pro-Russian separatists in eastern Ukraine, said a handful of Russian fighters, mostly of Armenian descent, had gone privately to Nagorno-Karabakh but had not stayed long.
"They understood quickly that they wouldn't be properly used there," he said.
Moscow and Ankara have cited the presence of foreign fighters as a threat to stability.
After Armenia reported two Syrian fighters had been captured, Russia estimated 2,000 mercenaries from the Middle East were fighting, and Erdogan said Armenia was using Kurdish militants. The reports have not been confirmed independently.
Political analyst Fyodor Lukyanov said Russian involvement was unlikely to match the support received by the separatists in eastern Ukraine and that even an "accidental hit" on Armenian territory would not be viewed by Moscow as "aggressive action".
Moscow has a pragmatic relationship with Ankara that has overcome past crises and both worry about the security of oil and gas pipelines in Azerbaijan.
But Razoux said deeper Russian involvement would be possible were Azerbaijan to attack Yerevan or Russia's military base in Gyumri, northwest of the capital.
"Russia has established a number of outposts along Armenia's borders to make a statement," he said. "But (Russian President Vladimir) Putin has made it equally clear that this protection does not, and never did, extend to where Armenia really wants support right now - in Nagorno-Karabakh."
More Aid Flights
Humanitarian aid deliveries from Russia to Armenia have increased in recent weeks, according to flight tracking data and two Armenian airline operators.
An IL-76 military cargo plane previously used by the Armenian postal service and now operated by Atlantis Armenian Airlines had flown almost daily between Russia and Yerevan since early October, the airline said.
On Friday, Reuters reporters saw the plane at Yerevan airport after a flight from Moscow, loaded with three minibuses, four winter tyres and dozens of sacks and boxes marked with red crosses. Labels of many of them identified Armenia's Moscow embassy as the sender.
Two Airbus A320 commercial jets operated by Atlantis European airlines have also been delivering humanitarian aid from southern Russia since early October.
On September 24, after the joint Turkish-Azeri military drill, flight tracking data showed three Turkish Air Force A400 heavy transport planes making a return trip to the Azeri capital Baku.
Turkey's support for Azerbaijan has had "a game-changing effect," said Laurence Broers, Caucasus programme director at the Chatham House think-tank.
"If the Azerbaijani advance proceeds as it has to date, there won't be a need for further Turkish involvement," he said, adding that Turkey "would probably lend increased support" if Azeri advances stalled.