Moscow on Monday expressed "serious concern" over fresh tensions between Armenia and Azerbaijan, which have been locked for decades in a deadly territorial conflict.
The comments from the Foreign Ministry came a day after Azerbaijan set up a checkpoint on the only land link to the contested region of Nagorno-Karabakh, sparking anger from Armenia.
Armenia and Azerbaijan have fought two wars over the mountainous enclave of Karabakh that have left tens of thousands dead.
The majority-ethnic Armenian region is still internationally recognized as part of Azerbaijan despite breaking away after the fall of the Soviet Union, with tensions regularly flaring between the two countries.
Moscow brokered a ceasefire after the latest bout of fighting in 2020, and posted peacekeepers along the sole road linking Karabakh to Armenia, the Lachin corridor.
"We express our serious concern about the situation within the zone of responsibility of Russian peacekeepers in Nagorno-Karabakh," the Russian foreign ministry said.
The statement also warned against "unilateral steps" in violation of the ceasefire.
Tensions over the Lachin corridor have soared since last year, with Armenia accusing Azerbaijan of blockading the route and creating a humanitarian crisis in the mountainous enclave.
Under the ceasefire agreement, Azerbaijan must guarantee safe passage through the corridor.
Azerbaijan, however, said it set up the checkpoint on Sunday "to prevent the illegal transportation of manpower, weapons, mines."
It added the checkpoint "shall be implemented in conjunction with the Russian peacekeeping force."
Armenia denied the claims as a "far-fetched and baseless pretext" and said the move breached the ceasefire.
The country of 3 million people on Monday commemorated the 108th anniversary of the Armenian genocide.
Clutching flowers and flags, thousands of Armenians gathered at the Tsitsernakaberd memorial, and many condemned Azerbaijan's latest move.
Vrej Sarkis, a 45-year-old doctor, accused Azerbaijan of "playing games" with Armenians' morale.
"They blocked this corridor so Armenians cannot come out from this region to visit their families in Armenia," he added.
Sos Kroyan, a 53-year-old lawyer, said Moscow's peacekeepers "are probably doing nothing, they are just surrendering checkpoint by checkpoint."
Armenia has accused Russia, which has been bogged down in Ukraine, of failing to fulfil its peacekeeping role.
Prime Minister Nikol Pashinyan, who laid flowers at the memorial, said Armenia was facing threats "exacerbated due to regional or extra-regional reasons."
Kremlin spokesman Dmitry Peskov said that "the situation is not easy, it requires additional efforts."
Analysts stress that Moscow does not want to hurt ties with Azerbaijan's patron Turkey over Armenia.
'Impotence or unwilligness'
Independent expert Arkady Dubnov told AFP there was already a "permanent irritation in Yerevan with regard to the actions of Russia."
He said Moscow was seen as showing "impotence or unwillingness to put pressure on Azerbaijan."
Since the collapse of the Soviet Union in 1991, Armenia has relied on Russia for its military and economic support.
The country is part of Russia-led regional Collective Security Treaty Organization (CSTO) and hosts a Russian military base.
Expert Andrei Suzdaltsev said the failure of Moscow's troops to stand up for Armenia in the conflict with Azerbaijan "sharply undermined the credibility of the CSTO."
In January, Armenia scrapped plans to host CSTO drills, but has so far refused to quit the pact altogether.
Many analysts say the small country cannot afford to abandon the CSTO, even as the United States and EU have sought to take the lead in peace talks.
"Armenia made a sharp political turn. It moved away from a united front with Moscow to stabilize the situation," Suzdaltsev said.
In March, Russian Foreign Minister Sergei Lavrov accused the West of "undisguised attempts... to undermine the region's security architecture."
"We see what goals the West pursues in the South Caucasus. It doesn't hide them — to tear Russia away" from the region, he said.