Armenia on Tuesday asked Russia to help defend its territorial sovereignty against Azerbaijan and said a heavy border clash between Azeri and Armenian forces was underway, Russia's TASS news agency reported.
Tensions between Yerevan and Baku remain high after a 44-day war last year between ethnic Armenian forces and the Azeri army, backed by Turkey, that killed at least 6,500 people and ended in a decisive victory for Azerbaijan.
The Azeri defence ministry spoke of what it said was a tense situation on the border with Armenia where it said a military operation was underway to repel what it called large-scale "provocations."
It said Armenian forces were shelling Azeri army positions with artillery and mortar fire and that it had destroyed some Armenian military hardware.
The Armenian defence ministry was cited by Russian news agencies as saying in a statement that Azerbaijan was using artillery, small arms and armour in ongoing battles.
TASS said four Armenian soldiers had been wounded.
"Since Azerbaijan has attacked Armenia's sovereign territory we are asking Russia to defend Armenia's territorial integrity based on an existing 1987 (mutual defence) agreement between our countries," the Interfax news agency cited Armen Grigoryan, the secretary of Armenia's Security Council, as saying.
Russia has a military base in Armenia as well as a peacekeeping force in the Nagorno-Karabakh enclave where last year's war unfolded.
There was no immediate response from Russia to the Armenian appeal.