Russia on Thursday accused the European Union of seeking to fuel "geopolitical confrontation" by establishing a new civilian mission in Armenia.
"The appearance of the EU representatives in the border regions of Armenia ... can only bring the geopolitical confrontation to the region and exacerbate existing contradictions," Russia's Foreign Ministry said in a statement.
The EU said the two-year mission, approved on Monday, would contribute to stability in border areas of Armenia and help ensure "an environment conducive to normalization efforts between Armenia and Azerbaijan."
Moscow is the main power broker in the South Caucasus and has long resented any meddling by outside powers in former Soviet republics, a zone it refers to as its "near-abroad." It deployed peacekeepers to the region in 2020 after a war between Armenia and Azerbaijan, their second confrontation since the 1991 collapse of the Soviet Union.
Its authority is being tested, however, as the war in Ukraine consumes the attention of the Kremlin and reduces its ability to fight fires elsewhere.
"The EU's attempts to gain a foothold in Armenia at any cost and to squeeze Russia's mediation efforts could damage the fundamental interests of Armenians and Azerbaijanis in their aspirations for a return to peaceful development in the region."
Russian peacekeepers and Russian border guards deployed on Armenia's borders will remain the key factor for regional stability for the foreseeable future, the ministry added.
The EU's move comes amid growing Armenian frustration over the failure of Russian peacekeepers to reopen the Lachin corridor, the only route linking Armenia to Karabakh.
The corridor has been blocked since Dec. 12, when environmental activists stopped traffic by setting up tents. Azerbaijan denies any blockade, saying the activists are staging a legitimate protest against illegal mining activity.
The Kremlin said on Tuesday Russia was working hard with both sides in the region to resolve the situation.
The United States threw its support behind the EU mission.
"We welcome efforts by partners, including the European Union, to build confidence in the region and to ensure an environment conducive to direct dialogue between Armenia and Azerbaijan," State Department spokesperson Vedant Patel told reporters.
U.S. Secretary of State Antony Blinken has also stepped up Washington's role in diplomacy between Armenia and Azerbaijan through regular talks with the two countries' leaders. Earlier this week he urged Azerbaijan to lift the blockade on the road into Karabakh.
Meanwhile, Azerbaijani President Ilham Aliyev rejected the accusation that the Armenian population in Karabakh faced a "humanitarian crisis" as "groundless, false, and absurd."
He said Russian peacekeepers and the Red Cross had ensured the delivery of civilian goods to Karabakh.
"Thousands of civilian cars have entered and left Karabakh since Dec. 12," he told a newly appointed French ambassador.
Relations between the former Soviet republics of Armenia and Azerbaijan have been tense since 1991 when the Armenian military illegally occupied Karabakh, a territory internationally recognized as part of Azerbaijan, and seven adjacent regions.
The clashes erupted on Sept. 27, 2020, with the Armenian Army attacking civilians and Azerbaijani forces, violating several humanitarian cease-fire agreements. During the 44-day conflict, Azerbaijan liberated several cities and around 300 settlements and villages that had been occupied by Armenia for almost 30 years.
The fighting ended with a Russian-brokered agreement on Nov. 10, 2020, which was seen as a victory for Azerbaijan and a defeat for Armenia.
However, the cease-fire has been broken several times since then.