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Moscow on Monday expressed "serious concern" over fresh tensions between Armenia and Azerbaijan and said any unilateral moves in violation of ceasefire agreements were unacceptable.

The comments came a day after Azerbaijan said it had set up a checkpoint on the only land link to the contested region of Nagorno-Karabakh, sparking an angry response from Armenia.

Armenia and Azerbaijan have fought two wars over Karabakh.

The latest fighting, in 2020 left more than 6,500 dead and ended with a Russian-brokered truce. But Armenia, which relies on Russia as a security guarantor, has accused Moscow of failing to fulfil its peacekeeping role.

"We express our serious concern about the situation within the zone of responsibility of Russian peacekeepers in Nagorno-Karabakh," the Russian foreign ministry said.

"We call on the parties to immediately return to the existing agreements," the ministry said in a statement.

The statement also pointed to "the unacceptability of any unilateral steps" in violation of the Moscow-brokered ceasefire and stressed the "lack of progress" in Russia-mediated talks between the arch-foes in recent months.

Experts say that Russia, which has been bogged down in Ukraine, does not want to hurt ties with Azerbaijan's patron Turkey over Armenia.

"The situation is not easy, it requires additional efforts," Kremlin spokesman Dmitry Peskov said.

"The above-mentioned countries need to understand that there is no alternative to the implementation of the mentioned agreement," Peskov said.

Under the Russia-brokered ceasefire, Azerbaijan is required to guarantee safe passage through the Lachin corridor.

On Sunday, however Azerbaijan said it set up the checkpoint "to prevent the illegal transportation of manpower, weapons, mines."

Armenia said setting up the checkpoint was "a gross violation" of the ceasefire agreement, part of Baku's "policy of ethnic cleansing in Nagorno-Karabakh."

Washington and Paris have also condemned the move.