Early on Sunday, Armenia announced it was declaring martial law, mobilising its army and ordering civilians to shelter. It claimed its neighbour Azerbaijan had launched a military operation inside a disputed region called Nagorno-Karabakh. Azerbaijan said it attacked only in response to Armenian shelling.

Nagorno-Karabakh is recognised internationally as Azerbaijan’s territory but has a mostly Armenian population who have resisted Azerbaijani rule for more than a century. In 1991 the region declared independence and since then it has ruled itself – with Armenian support – as the unrecognised Republic of Artsakh.

Despite signs in the past two years of possible progress towards peace, one of Europe’s “frozen conflicts” has erupted again. Since Sunday, forces from Nagorno-Karabakh along with the Armenian military have been fighting Azerbaijani troops, armour and aircraft. At least two dozen people have been killed including civilians, and hundreds more are said to be injured. Azerbaijan has claimed to have taken territory inside Nagorno-Karabakh, a claim the Armenians dispute, and it appears to be a fluid situation on the ground.

Nagorno-Karabakh, a mountainous, landlocked region inside the borders of Azerbaijan, has been a source of dispute since before the creation of the Soviet Union. Tensions were suppressed when both Armenia and Azerbaijan were Soviet states, but they re-emerged as the cold war ended and Communist party control of the bloc dissolved.