CLASHES erupted between Azerbaijani and Armenian troops, Russian news agencies reported early on Tuesday, in a resumption of decades-old hostilities linked to the disputed territory of Nagorno-Karabakh.

Azerbaijan, which re-established full control over the territory in a six-week conflict in 2020, acknowledged casualties among its forces. Armenia made no mention of losses, but said clashes persisted overnight.

The Yerevan government said it would invoke a cooperation agreement with Russia and appeal to a Russia-led security bloc, the Collective Security Treaty Organisation, as well as the United Nations Security Council, Interfax reported.

In addition to Russian President Vladimir Putin, Armenian Prime Minister Nikol Pashinyan has called French President Emmanuel Macron and United States Secretary of State Antony Blinken to discuss the situation.

Blinken urged an immediate end to hostilities for which each side has blamed the other.

"Several positions, shelters and reinforced points of the Azerbaijan armed forces ... came under intense shelling from weapons of various calibres, including mortars, by units of the Armenian army," the agencies quoted a statement by Azerbaijan's Defence Ministry as saying.

"As a result, there are losses in personnel and damage to military infrastructure."

Azerbaijani statements said Armenian forces had been engaged in intelligence activity on its border, moved weapons into the area and on Monday night had conducted mining operations.

It said its actions were "strictly local in nature aimed at military targets."

Armenia's Defence Ministry said: "Intensive shooting is continuing - started as a result of a large-scale provocation by the Azerbaijani side. Armenia's armed forces have launched a proportionate response."

Conflict first broke out in the late 1980s when both sides were under Soviet rule and Armenian forces captured swathes of territory near Nagorno-Karabkah - long recognised internationally as Azerbaijan's territory, but with a large Armenian population.

Azerbaijan regained those territories in the 2020 fighting, which ended with a Russian-brokered truce and thousands of residents returning to homes from which they had fled.

The leaders of both countries have since met several times to hammer out a treaty intended to establish a lasting peace.