Russia expressed "serious concern" amid an ongoing escalation of tensions between Azerbaijan and Armenia over Karabakh.

The comments from the foreign ministry came a day after Azerbaijan set up a checkpoint on the only land link to Karabakh, sparking anger from Armenia.

Armenia and Azerbaijan have fought two wars over Karabakh, which had been under Armenian occupation for three decades.

Moscow brokered a cease-fire 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 Karabakh," the Russian foreign ministry said.

The statement also warned against "unilateral steps" in violation of the cease-fire.

Under the cease-fire 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 cease-fire.

Kremlin spokesperson 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 Türkiye over Armenia.

'Impotence or unwillingness'

Independent expert Arkady Dubnov told Agence France-Presse (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 the 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.