Azerbaijan on Sunday set up a checkpoint on the only land link between Armenia and the Karabakh region.
"The units of the Azerbaijani Border Service established a border checkpoint on the sovereign territory of Azerbaijan, at the entrance of the Lachin-Khankendi road," the state border service said.
Under the Russia-brokered cease-fire that ended the 2020 conflict, Azerbaijan is required to guarantee safe passage on the Lachin corridor, which is patrolled by Russian peacekeepers.
Azerbaijan said it set up the checkpoint at 12 p.m. (8 a.m. GMT) on Sunday "to prevent the illegal transportation of manpower, weapons, mines."
It added the checkpoint "shall be implemented in interaction with the Russian peacekeeping force."
Washington said it was "deeply concerned that Azerbaijan's establishment of a checkpoint on the Lachin corridor 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 and call on the parties to resume peace talks and refrain from provocations and hostile actions along the border," the State Department said in a statement.
A French Foreign Ministry statement said it "deplores" the development claiming it is a violation of the cease-fire agreements that would hamper the negotiation process.
Since last year tensions have risen over the Lachin corridor, with Russia focused on its offensive in Ukraine.
In December, Azerbaijani activists blocked the Lachin corridor to protest illegal mining.
Azerbaijan built the checkpoint following the "threats and provocations" from Armenia.
Yerevan has been using the corridor for the rotation of army staff, "the transfer of weapons and ammunition, entrance of terrorists, as well as illicit trafficking of natural resources and cultural property."
Azerbaijan recorded military convoys entering its territory and "the construction of military infrastructure... at the point closest to the territory of Azerbaijan."
Yerevan, which relies on Russia as a security guarantor, has grown frustrated over what it sees as Kremlin's failure to fulfill its peacekeeping role.
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 for three decades.
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 Armenia had occupied 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 violated several times since then.
Several servicemen from both sides have been killed in clashes in recent months.
