Russia expressed concern on Thursday over escalating tensions between Armenia and Azerbaijan as a key road linking Armenia to the disputed Nagorno-Karabakh enclave remained blocked for the fourth day.

The two countries have fought repeated wars over Nagorno-Karabakh – internationally recognised as part of Azerbaijan but home to about 120,000 ethnic Armenians – since the 1991 collapse of the Soviet Union. As recently as September, more than 200 soldiers were killed in a flare-up of fighting.

A group of Azerbaijanis claiming to be environmental activists blocked the Lachin corridor, the only land route for people, goods, food and medical supplies to reach Nagorno-Karabakh from Armenia across Azerbaijani territory, at the start of this week.

Armenian Prime Minister Nikol Pashinyan said on Thursday that the closure of the passage was a “gross violation” of a 2020 peace agreement between Baku and Yerevan and that the population of the enclave had been made into hostages.

Armenia says the protesters have been dispatched by the Azerbaijani government in an attempt to block Armenia’s access to the region. Baku rejects those claims, saying the dispute is over illegal Armenian mining in Nagorno-Karabakh.

The standoff is a test of Russia’s authority as the main security guarantor in the region at a time when its struggles in the war in Ukraine risk undermining its top-dog status among former Soviet republics in the South Caucasus and Central Asia.

The Azerbaijani protesters have managed to blockade the road despite the presence of Russian peacekeepers from a 5,000-strong force that Moscow sent in late 2020.

Russian Foreign Ministry spokeswoman Maria Zakharova expressed Moscow’s concern over the situation and said it expected the route to be cleared soon.

“Russia’s defence ministry and the Russian peacekeeping contingent have been actively working to de-escalate the situation and we expect full transport links to be restored in the very near future,” Zakharova told reporters.

Russia is an ally of Armenia through a mutual self-defence pact, but tries to maintain warm relations with Azerbaijan and has rejected calls by Yerevan to provide military support.

The U.S. State Department said on Tuesday that the closure of the Lachin corridor “has severe humanitarian implications and sets back the peace process”, and called on Azerbaijan to restore free movement.

The European Union also expressed “serious concern” over the deteriorating situation, urging Azerbaijan to restore free and secure passage.