Tensions have risen as Azerbaijan sets a checkpoint on the route between Armenia and the disputed Nagarno-Karabakh region violating an agreement on Russian control over the 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 on Sunday.

The measure was followed by mutual claims of border shootings by Azeri and Armenian forces.

Karabakh is internationally recognized as part of Azerbaijan, but has a primarily Armenian population that has resisted Azerbaijani rule since a separatist war there ended in 1994.

In 2020, the second Karabakh war broke out, killing more than 6,500 people on both sides during a six-week conflict. The war ended with a Russian-brokered deal that saw Yerevan cede swathes of the Azerbaijani territory that it had been holding for several decades.

Armenia said the checkpoint on the Hakari bridge in the Lachin corridor was a gross violation of the 2020 ceasefire agreement.

It called on Russia to enforce the agreement, which states that the Lachin corridor must be under control of Russian peacekeepers.

"We call on the Russian Federation to ultimately implement the trilateral statement," Armenia's Foreign Ministry said of the agreement that was brokered by Russian President Vladimir Putin.

In late February, the UN's top court ordered Azerbaijan to allow free passage through the Lachin Corridor. The International Court of Justice cited "shortages of food, medicines, and other life-saving medical supplies" that effectively deprived ethnic Armenians in the area of crucial care. 

Baku must "take all measures at its disposal to ensure the unimpeded movement of persons, vehicles, and cargo along the Lachin Corridor in both directions," the court said.

It came after Azerbaijan's environmental activists picketed the corridor, demanding Armenia stop mining gold and copper-molybdenum deposits in Karabakh, which officials in Baku claimed Armenians are exporting illegally.

Armenia accused the activists of acting with official Azerbaijani support. Baku officials denied they were behind the campaign.

The Sunday development also came after Armenia said last month that Azerbaijan was preparing an escalation against Yerevan via the use of an "aggressive rhetoric" that was aimed at undermining the existing agreements between the two countries. Yerevan also alleged that Baku was preparing for what it called subjecting the Armenians of Nagorno-Karabakh to "genocide" and for "a new aggression" against Armenia.

Addressing the issue of the checkpoint, the Azeri Foreign Ministry alleged, "Providing border security, as well as ensuring safe traffic on the road, is the prerogative of the government of Azerbaijan, and an essential prerequisite for national security, state sovereignty, and the rule of law."

Pictures of the bridge posted on social media by Azeri officials showed one side of it blocked by vehicles and soldiers.

Meanwhile, Armenia's Defense Ministry claimed that a soldier had been killed at around 0750 GMT when Azeri forces opened fire on an Armenian position in Sotk, a village east of Lake Sevan. Azerbaijan, at first, denied its forces had killed the soldier. Later on, however, Baku claimed that Armenian soldiers fired on Azeri units at around 1110 GMT in the Lachin district, a claim Armenia denied.

The US expressed "deep concern" after Azerbaijan established the checkpoint on the road, saying the measure undermines efforts toward peace in the region. The State Department also said there should be free and open movement of people and commerce on the Lachin corridor, urging both sides to resume peace talks.