April 23 (UPI) — Azerbaijan on Sunday established a checkpoint on the Lachin corridor, the sole land link between Armenia and the contested Nagorno-Karabakh region, sparking outrage in Yerevan and concern in Washington, D.C.

The Nagorno-Karabakh region has been a flashpoint for the two countries for decades. Though internationally recognized as part of Azerbaijan, the contested border area is predominately populated with Armenians and had declared independence from Baku in early 1991.

Two wars have been fought over the region by Azerbaijan and Armenia, with the most recent being a six-week conflict in 2020 that ended with a cease-fire agreement that included the maintenance of the Lachin corridor, which is a mountain road and the only link for residents of Nagorno-Karabakh to Armenia.

Baku’s state border agency said Sunday it established the border crossing in response to Armenia using the road to transport soldiers, ammunition, landmines and facilities to erect its own checkpoint in Azerbaijani territory.

Its ministry of defense accused Armenia of having used the corridor on Saturday to transport two military-purpose containers and a convoy of military vehicles into the Nagorno-Karabakh region for this purpose.

A shot clip from surveillance footage of the purported transport of the containers across the corridor was published on Twitter by the Azerbaijan ministry of defense, which said in a statement that the movement occurred at 8 p.m., local time, Saturday, with support of Russian peacekeepers.

The Azerbaijani checkpoint establishes a “border control mechanism” to increase transparency and counter these “threats and provocations” that violated the November 2020 agreement, Azerbaijan’s foreign ministry said in a separate statement.

The Armenia foreign ministry retorted by calling the checkpoint a gross violation of the Trilateral Statement “aimed at the consistent implementation of Azerbaijan’s policy of ethnic cleansing in Nagorno-Karabakh and the complete annihilation of Armenians from Nagorno-Karabakh.

“This, as well as the previous similar actions of Azerbaijan, combined with continuous Armenophobia and threatening rhetoric, are aimed at scuttling the negotiations on the document of the normalization of relations between Armenia and Azerbaijan,” it said in a statement.

Meanwhile, in Washington, D.C., State Department principal deputy spokesman Vedant Patel said the administration of President Joe Biden is “deeply concerned” about the establishment of the checkpoint as it “undermines efforts to establish confidence in the peace process.

“We reiterate that there should be free and open movement of people and commerce on the Lachin corridor on the parties to resume peace talks and refrain from provocations and hostile actions along the border,” he said.

The checkpoint was erected following months of growing hostility between the two sides that has seen both incur casualties.

On Sunday, Armenia’s ministry of defense announced that at 11:50 a.m., service member Artyom Poghosyan was “lethally wounded” by Azerbaijani forces.