Here are the key issues surrounding the conflict involving the two former Soviet republics in the Caucasus.
At the heart of the standoff between Yerevan and Baku is the contested Nagorny Karabakh region. The Soviet authorities merged the predominantly ethnic Armenian territory with Azerbaijan in 1921.
After the 1991 collapse of the Soviet Union, Armenian separatists seized it in a move supported by Yerevan. An ensuing war left 30,000 dead and forced hundreds of thousands from their homes.
Despite a ceasefire mediated in 1994 by Russia, the United States and France, peace negotiations struggle to move forward and fighting erupts frequently.
The latest clashes on Sunday saw Azerbaijan and Armenian separatists accuse each other of igniting the fighting that left both sides with casualties, including civilians.
It followed a flare-up along the border in July which claimed the lives of 17 soldiers from both sides.
In April 2016, some 110 people were killed in the most serious fighting in years.