Armenia- Azerbaijan conflict explained: Why the former Soviet Republics are at the brink of war and how is Turkey involved?

Armenia and Azerbaijan, two ex-Soviet republics in the Caucasus, are locked in a decades-long territorial dispute with deadly fighting erupting Sunday. Here are the key issues surrounding their conflict.

Armenia and Azerbaijan, two ex-Soviet republics in the Caucasus, are locked in a decades-long territorial dispute with deadly fighting erupting Sunday. Here are the key issues surrounding their conflict. (Photos | AFP) 

WHY THE CONFLICT? At the heart of the standoff between Armenia and Azerbaijan is the contested Nagorny Karabakh region. The Soviet authorities merged the predominantly ethnic Armenian territory with Azerbaijan in 1921. IN PIC: Solders of Azerbaijan's breakaway region of Nagorny Karabakh march in the capital of Stepanakert on May 9, 2012, during a military parade.(Photo | AFP)

Mostly mountainous Nagorno-Karabakh — a region around 4,400 square kilometres (1,700 square miles) or about the size of the US state of Delaware — lies 50 kilometres (30 miles) from the Armenian border. IN PIC: Mobile artillery units of the Nagorno-Karabakh forces.(Photo | AFP)

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. IN PIC: An Armenian woman cries on the grave of her son Nagorny Karabakh who was killed in the Azeri-Armenian war. Photo dated: 12/06/1994. (Photo | AFP)

Despite a ceasefire mediated in 1994 by Russia, the United States and France, peace negotiations struggle to move forward and fighting erupts frequently. IN PIC: Armenian militiamen fire on Azeri positions near the village of Hanatag in Nagorny Karabakh 19 May 1992. (Photo | AFP)

In April 2016, some 110 people were killed in the most serious fighting in years. IN PIC: Armenians pay their respects to the coffins following the downing of military helicopter by Azerbaijani forces. (Photo | AFP)

Armenia (on left), a Christian country since the fourth century, has been rocked by political and economic instability since it gained independence from the former USSR. Muslim-majority Azerbaijan (on right), on the Caspian Sea, has been under the authoritarian grip of a single-family since 1993. (Photos | AFP)

ARE FOREIGN POWERS INVOLVED? Turkey, with ambitions to be regional powerbroker in the Caucasus, has thrown its weight behind oil-rich and Turkic-speaking Azerbaijan. Their alliance is fuelled by a mutual mistrust of Armenia, and Ankara routinely issues strongly-worded statements in support of Baku's ambitions to reclaim Nagorny Karabakh. IN PIC: Demonstrators hold national flags of Azerbaijan and of Turkey. (Photo | AFP)

Armenia harbours hostility towards Turkey over the massacres of some 1.5 million Armenians by Turkey under the Ottoman Empire during World War I. More than 30 countries have recognised the killings as genocide, though Ankara fiercely disputes the term. IN PIC: Armenian-Americans demonstrate ahead of a reception to Azerbaijan President Geidar Aliyev. (Photo | AFP)

Russia, which maintains close ties with Armenia, is the major powerbroker in the region. It leads the Collective Security Treaty Organisation military alliance of ex-Soviet countries that includes Armenia. Yerevan relies on Russian support and military guarantees because its defence budget is overshadowed by Azerbaijan's spending on arms. IN PIC: Russian servicemen take part in a military exercise. (Photo | AFP)

WHY THE SUDDEN UNREST? 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. (Photo | AP)

The Azerbaijani Defense Ministry claimed that Armenian forces started shelling the town of Tartar on Monday morning, while Armenian officials said the fighting continued throughout the night and Baku resumed “offensive actions' in the morning. IN PIC: Azeri troops conducting a combat operation against Armenia. (Photo | AFP)

Azerbaijan's Defense Ministry told the Interfax news agency Monday that over 550 Armenian troops have been killed, a claim that Armenian officials denied. (Photo | AP)