Moscow and Baku on Tuesday criticized the deployment of an expanded EU monitoring mission to Armenia's volatile border with Azerbaijan.
Arch Caucasus foes Armenia and Azerbaijan have fought two wars for control of the Nagorno-Karabakh border region but have recently made progress toward a peace deal.
The EU announced last week it had deployed a 100-member civilian mission (EUMA) for two years to the Armenian side of the frontier.
The move comes amid growing Western engagement in the ex-Soviet region viewed by the Kremlin as inside its sphere of influence.
"The EU is openly abusing its relations with Armenia and Azerbaijan," Russian Foreign Minister Sergei Lavrov told a news conference in Baku alongside his Azerbaijani counterpart Jeyhun Bayramov.
He added that the EU mission "raises doubts in terms of its legitimacy... functions, mandate and duration."
Requested by Armenia, the border mission expanded a 40-member team that was deployed for two months late last year.
"Armenia's position at normalization talks with Baku has become more destructive after the EU mission had been set up," Bayramov said.
He said the previous two-month mission "failed to achieve its goals."
Last week, Azerbaijani President Ilham Aliyev said that after what he called a "constructive" meeting with Armenian premier Nikol Pashinyan in Munich, Baku was studying Armenian proposals for a full peace treaty, which Yerevan had unveiled days before.
When the Soviet Union collapsed in 1991, ethnic Armenian separatists in Karabakh broke away from Azerbaijan. The ensuing conflict claimed around 30,000 lives.
Another flare-up in violence in 2020 left more than 6,500 dead and ended after a Russian-brokered truce that saw Armenia cede territories it had controlled for decades.